Fix cleanup task race in subscribe_track

[kixelated]

Summary

• The spawned unused() cleanup task removes the producer entry by track name, which could evict a newer producer inserted at the same key after a stale one was replaced (race from Fix stale TrackProducer returned from cache #945)
• Guard the removal with an is_clone() identity check so only the original producer instance is evicted; if a different producer now occupies the slot, reinsert it

Test plan

• cargo test -p moq-lite — all 142 tests pass
• Existing stale_producer and requested_unused tests cover the relevant paths

🤖 Generated with Claude Code

[coderabbitai]

Walkthrough

The change updates BroadcastConsumer::subscribe_track in the broadcast module to alter cleanup when a tracked producer becomes unused. The code now acquires the shared state lock and, instead of unconditionally removing the entry, conditionally reinserts a current producer if it exists and is not the same clone being dropped. No public APIs were changed.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main fix: addressing a race condition in the cleanup task of the subscribe_track function.
Description check	✅ Passed	The description comprehensively explains the race condition, the solution using is_clone() check, and confirms testing with all 142 tests passing.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/cleanup-task-race

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