Cannot return a 4xx status code from BedrockAgentCoreApp entrypoint

[markeramzn]

Describe the bug
Only status code 200 or 500 can be sent to a client when you use an entrypoint decorated function to handle requests for a BedrockAgentCoreApp. There's no way to send a status code in the 400s to indicate invalid user input.

To Reproduce
Run this server and send a request to the /invocations endpoint with body {}

from bedrock_agentcore.runtime import BedrockAgentCoreApp
from starlette.exceptions import HTTPException

app = BedrockAgentCoreApp()

@app.entrypoint
def invoke(payload):
    prompt = payload.get("prompt")
    if prompt is not None:
        return {"message": "a message"}
    else:
        raise HTTPException(status_code=400, detail="Prompt missing")

if __name__ == "__main__":
    app.run()

Expected behavior
Server should respond with a 400 status code to the client but instead it responds with a 500 status code.

Version (consider adding this to the template)
0.1.0

Additional context
Starlette supports either throwing an HTTPException with a status code or returning an instance of a Response or Response subclass like JSONResponse with a status code. Neither method works here because the endpoint function's return value gets converted to JSON and wrapped in a JSONResponse with status code 200 and exceptions are caught and replaced by the framework.