fix: suppress ClientConnectionError in frames() generator teardown

[livepeer-tessa]

Summary

Fixes daydreamlive/scope#897 — asyncio logs Task exception was never retrieved on every session close where media output is active.

Root Cause

In MediaOutput.frames(), the async generator's finally block cancels and awaits producer_task. During teardown, that task may raise aiohttp.ClientConnectionError('Connection closed') because the underlying connection was already dropped when the client disconnected. This error was not suppressed, causing asyncio to log it as an unhandled task exception.

Fix

Add aiohttp.ClientConnectionError to the suppress() context in the finally block, alongside the existing asyncio.CancelledError:

# before
with suppress(asyncio.CancelledError):
    await producer_task

# after
with suppress(asyncio.CancelledError, aiohttp.ClientConnectionError):
    await producer_task

Both errors are expected during normal session teardown — the connection being closed is the reason we're tearing down in the first place.

Impact

• Eliminates log noise (asyncio - ERROR - Task exception was never retrieved) on every clean session close
• No functional change — the session is already terminating when this fires
• aiohttp is already a declared dependency (>=3.9.0), so no new deps added

Testing

The error occurs on every session close with active media output. After this fix, the log line should no longer appear in Grafana/Loki for normal session teardowns (only genuine errors should remain).