[backend] No stack traces logged for error debugging

[athewsey]

When testing the agent locally (with uv run run.py), I'm not seeing useful stack traces when calls to /invocations error out: Just INFO: 127.0.0.1:50026 - "POST /invocations HTTP/1.1" 500 Internal Server Error

When Langfuse is configured correctly, I am seeing a little more information appear there - but not really enough to help track down errors especially when starting out. For example, when my LiteLLM gateway URL was mis-configured it just gave "Connection error." which was not particularly helpful.

At the moment I'm working around this with a temporary try/except block like this in main.py:

    @app.post("/invocations")
    async def invocations(request: dict, http_request: Request):
        try:
            ...
        except Exception:
            logger.exception("Oopsies!")
            raise

...But I think we should tweak the default logging config to make it a bit more debugging-friendly, especially when running locally.