stream: fix firefox datachannel readiness counting

[kfatehi]

Summary

I used an AI assistant to help trace this, then verified and tested the fix manually.

Fixes a Firefox-specific readiness bug in WebRTCBaseStream._parse_incoming_streams that prevented messaging (e.g. joystick commands) from being set up.

Root cause

expected_number_of_incoming_media was over-counted when SDP included a datachannel media section (m=application) with a=sendrecv (common in Firefox).

The parser counted that application media twice:

1. in the generic sendonly/sendrecv media count
2. again in the explicit datachannel (m.kind == "application") count

That made expected_number_of_incoming_media larger than the actual runtime media/events, so readiness never completed.

Fix

• Restrict generic sending-media counting to audio and video only.
• Count datachannel presence as one messaging channel (int(len(channel_medias) > 0)), avoiding double-counting.

Behavior

• Before: joystick/datachannel flow could work in Chrome but stall in Firefox.
• After: readiness completes correctly and messaging works in both Chrome and Firefox.

Tests

• Added regression test:
  • test_parse_incoming_streams_datachannel_not_double_counted
• Command run:
  • PYTHONPATH=. pytest -q -o addopts='' tests/test_stream.py -k datachannel_not_double_counted
  • 1 passed