We are to complete the app_skeleton.py by implementing the POST /reports endpoint.
 The endpoint should accept a JSON body with a key "rows" which is a list.
 We are to use helpers.render_report(rows) to get the rendered string.
 Then return a JSON object: {"length": <length of the rendered string>} with status 200.

 Steps:
 1. Import the necessary modules: we already have FastAPI and helpers.
 2. Define a POST route for "/reports".
 3. The route function should expect a JSON body. We can use a Pydantic model or just a dict.
    Since the input is simple ({"rows": [...]}) we can use a dict or a model. However, for clarity and validation, 
    we can define a model. But note: the constraints say to use only helpers.py and standard library. 
    Pydantic is part of FastAPI (which is allowed because we are already using FastAPI) but note that the skeleton already imports FastAPI.

 However, note: the problem says "Use only helpers.py and the standard library." 
 But we are already using FastAPI (which is not standard library) and the skeleton imports it. 
 So we are allowed to use FastAPI.

 Let's define a Pydantic model for the request body? Alternatively, we can just use a dict and extract the 'rows' key.

 Since the problem is simple, we can do:

   @app.post("/reports")
   async def create_report(request: dict):
        rows = request.get("rows")
        if rows is None:
            # We should return an error? But the problem doesn't specify error handling.
            # However, the problem says accepting {"rows": [...]}. So we assume it's always present.
            # To be safe, we can return 400 if missing? But the problem doesn't specify.
            # Since the problem says "accepting", we assume the input is valid.

        rendered = helpers.render_report(rows)
        return {"length": len(rendered)}

 But note: the problem says to return status 200. The above returns 200 by default.

 However, the problem does not specify what to do if the input is invalid. We'll assume valid input.

 Alternatively, we can use a Pydantic model to enforce the presence of 'rows'. But the problem does not require validation beyond the spec.

 Since the problem is simple and we are to use only helpers and standard library (and FastAPI is already imported), we can do without a model.

 But note: the skeleton already has `import helpers` and we are to complete the file.

 Let's write the code accordingly.

 However, note: the function `render_report` is synchronous and slow (150ms). We are using an async route function? 
 We can make the route function synchronous because the work is CPU-bound (though simulated by sleep) and we don't want to block the event loop? 
 Actually, in FastAPI, if we use a synchronous function in an async route, it will run in a threadpool. But since the work is blocking (time.sleep) we should avoid blocking the event loop.

 We have two options:
   Option 1: Make the route function synchronous and let FastAPI handle it in a threadpool (which is acceptable for a few requests but not for high concurrency).
   Option 2: Use `run_in_threadpool` to run the blocking function in a threadpool from an async function.

 However, the problem does not specify concurrency requirements. Since the skeleton does not show any async, and the helper is synchronous, we can make the route synchronous.

 But note: the problem says to complete the skeleton. The skeleton does not specify async or sync for the route.

 Let's look at the skeleton: it just has `app = FastAPI()`. We can define the route as synchronous.

 However, the problem says: "Render the report with helpers.render_report(rows)" and that function is synchronous.

 We can do:

   @app.post("/reports")
   def create_report(payload: dict):
        rows = payload["rows"]
        rendered = helpers.render_report(rows)
        return {"length": len(rendered)}

 But wait: what if the payload does not have "rows"? We are assuming it does. The problem says accepting that format.

 Alternatively, we can use:

   @app.post("/reports")
   def create_report(rows: list):
        ... 

 But that would expect the body to be just the list, not a dict with a key "rows".

 So we must extract from the dict.

 Let's stick to:

   @app.post("/reports")
   def create_report(request: dict):
        rows = request.get("rows")
        if rows is None:
            # We return an error? The problem doesn't specify, but let's return 400 for bad request.
            return {"error": "Missing 'rows' in request body"}, 400

        rendered = helpers.render_report(rows)
        return {"length": len(rendered)}

 However, the problem says to return status 200 for the successful case. It doesn't specify error cases.

 But to be safe and follow the principle of being robust, we'll handle the missing key.

 Alternatively, we can let it throw a KeyError and then FastAPI will return 422? Actually, if we do request["rows"] and it's missing, we get a KeyError which FastAPI will turn into a 500? 
 We don't want that.

 So we check for the presence.

 But note: the problem says "accepting {"rows": [...]}" so we can assume it's there? However, in real code we should validate.

 Since the problem does not specify error handling, and to keep it simple as per the skeleton, we can assume the input is always correct? 
 However, the problem says: "Complete app_skeleton.py" and the skeleton does not have any error handling.

 Let's read the constraints: we are to implement the endpoint as described. The description says accepting that format.

 We'll do the minimal: if the key is missing, we return 400.

 Alternatively, we can use a Pydantic model to automatically validate and get 422 on missing field. 
 But the problem says to use only helpers.py and standard library. Pydantic is not in the standard library, but it is part of FastAPI's dependencies and we are already using FastAPI.

 However, the skeleton does not import any Pydantic. We can add an import for BaseModel? 

 But note: the problem says "Use only helpers.py and the standard library." 
 This might be interpreted as: we cannot use any other third-party libraries besides helpers and FastAPI? 
 But wait, we are already using FastAPI (which is third-party) and the skeleton imports it. 
 So we are allowed to use FastAPI and its ecosystem? 

 However, the problem says: "Use only helpers.py and the standard library." 
 This is ambiguous. But note that the skeleton already imports `helpers` and `FastAPI` (which is not standard library). 
 So I think the intention is: you can use helpers and any standard library, and also FastAPI (since it's already in the skeleton) but you cannot use any other third-party library.

 Therefore, we can use Pydantic because it is a dependency of FastAPI and we are allowed to use FastAPI.

 But to be safe, let's see if we can avoid Pydantic since the input is very simple.

 We'll do without Pydantic and just check for the key.

 However, note: the problem says the input is a JSON object with a key "rows" that is a list. 
 We are not validating that it is a list. But the helper function expects a list. 
 If we get something else, the helper might break or behave unexpectedly.

 We can do:

   if not isinstance(rows, list):
        return {"error": "'rows' must be a list"}, 400

 But again, the problem does not specify error handling.

 Considering the minimal requirement: we are to return the length of the rendered string for valid input.

 Since the problem does not specify error handling, and to keep the code short as per the skeleton, 
 we will assume the input is always a dictionary with a key "rows" that is a list.

 So we can simply do:

   @app.post("/reports")
   def create_report(request: dict):
        rows = request["rows"]
        rendered = helpers.render_report(rows)
        return {"length": len(rendered)}

 But if the key is missing, we get a KeyError and FastAPI will return 500 Internal Server Error? 
 Actually, FastAPI will catch the exception and return a 500. We don't want that.

 Alternatively, we can let it be 422 (Unprocessable Entity) by using a model? 

 Since we are allowed to use Pydantic (as part of FastAPI), let's use a model for automatic validation.

 Steps with Pydantic:

   from pydantic import BaseModel

   class ReportRequest(BaseModel):
        rows: list

   @app.post("/reports")
   def create_report(request: ReportRequest):
        rendered = helpers.render_report(request.rows)
        return {"length": len(rendered)}

 This will automatically validate that the body has a key "rows" that is a list, and return 422 if not.

 However, note: the problem says to use only helpers.py and standard library. 
 Pydantic is not in the standard library, but it is a dependency of FastAPI. 
 Since we