fix: prevent duplicate stream terminal events

[shhahbaz]

Changes

• Make PCM stream terminal handling idempotent.
• Prevent duplicate end/error notifications when streams emit both terminal and close events.
• Make active stream cleanup safe when cancellation is followed by a late close event.
• Track cleaned stream entries explicitly.
• Count cancellation exactly once.
• Add regression tests for end/close, error/close, and cancellation races.

Why

Readable streams commonly emit end followed by close. The worker previously attached terminal handlers to both events without guarding against repeated execution. This could send duplicate stream completion frames, increment lifecycle counters incorrectly, and run cleanup more than once.

Cancellation had a related race where the stream was cleaned immediately, then a later close event performed terminal handling again.

This fix ensures each stream has exactly one terminal outcome and exactly one cleanup operation without changing the Lavalink-compatible stream protocol.

Checkmarks

• The modified endpoints have been tested.
• Used the same indentation as the rest of the project.
• Still compatible with LavaLink clients.

Additional information

• No new test framework or runtime dependency was introduced.
• The finalizer is limited to worker PCM stream lifecycle handling.

[1Lucas1apk]

Thanks for your contribution :P