fix(rtc): handle repeated local track subscription events

[swayamg20]

Fixes #770

Problem

During a full reconnect, the server can resend local_track_subscribed for a reused LocalTrackPublication. Its _first_subscription future is one-shot and already resolved, so calling set_result(None) again raises asyncio.InvalidStateError. _listen_task catches the exception, logs an error, and drops the repeated event.

Fix

Only resolve _first_subscription while it is pending. The handler still emits local_track_subscribed for every event, preserving callback behavior across reconnects.

The regression test dispatches the same event twice and verifies both branches:

• the first event resolves wait_for_subscription();
• the repeated event does not raise and is still delivered to listeners.

Testing

• Regression test fails with InvalidStateError on the unguarded handler and passes with this change.
• MACOSX_DEPLOYMENT_TARGET=11.0 uv run pytest livekit-rtc/tests -q — 33 passed, 14 skipped.
• Ruff lint and format checks pass across the maintained Python packages and tests.
• Mypy passes across livekit-protocol, livekit-api, and livekit-rtc/livekit.
• make build-wheel PACKAGE=livekit-rtc succeeds.

The full root test command also reached 48 passed and 30 skipped. Three audio-fixture tests could not run locally because Git LFS was unavailable and the WAV fixtures remained pointer files; CI is configured to fetch those LFS assets.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.

[cloudwebrtc]

lgtm