fix(livekit): treat a negative ADM device count as zero devices

[RaphaelFakhri]

Summary

PlatformAudio::recording_devices() and playout_devices() iterate over usize::MAX indices when the audio device module reports a negative device count.

The module returns a negative value (-1) when it is not initialized or fails to enumerate. recording_device_count() and playout_device_count() cast that i16 to usize, which wraps to 18446744073709551615. The iterator then walks every index at 100% CPU. This is the symptom described in #1468, where the platform ADM is left uninitialized after the last peer connection is released.

Changes

• Add device_count, which maps a negative count to zero, and use it for both counts.
• Add unit tests for negative and non-negative counts.
• Add a livekit patch changeset.

This does not change the cause of the negative count in #1468. It makes the API return no devices instead of spinning.

Testing

cargo test -p livekit --lib device_count

• Before the fix, negative_device_count_is_zero fails (1 passed, 1 failed).
• After the fix, both tests pass (2 passed).
• cargo fmt -p livekit -- --check reports no changes.

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