New Ruff failure

[rwb27]

Ruff (in our unpinned tests) flags a new warning:

error[ASYNC119]: Yield in context manager in async generator may not trigger cleanup
   --> src/labthings_fastapi/outputs/mjpeg_stream.py:292:21
    |
290 |                 i = await self.next_frame()
291 |                 async with self.buffer_for_reading(i) as frame:
292 |                     yield frame
    |                     ^^^^^^^^^^^
293 |             except StopAsyncIteration:
294 |                 break
    |
help: Use `@asynccontextmanager` if appropriate, or refactor

This is not an @asynccontextmanager and shouldn't be: I guess it probably is the case that we may want to consider refactoring.

To be honest, the buffer-retention mechanism doesn't serve much purpose so we might be better off just simplifying the stream code, but either way it probably wants looking at.