fix(audio): unblock chunked reader close during chunk prefetch

[gjermundgaraba]

This PR fixes a shutdown race in the audio chunked reader that I noticed in the CI run for another PR I opened: #302

Changes are essentially:

• switch chunked reader shutdown to context-based cancellation
• cancel in-flight HTTP requests and retry backoff when Close() is called
• remove per-chunk error state and simplify chunk fetch coordination
• prevent prefetch goroutines from being started after shutdown begins
• make ReadAt fail fast with net.ErrClosed once the reader is closed
• make latency metric collection concurrency-safe

• unit and integration tests

[Copilot]

Pull request overview

Fixes shutdown races and close behavior in the audio HTTP chunked reader by switching to context-based cancellation, ensuring in-flight requests/backoff are cancelable, and making latency metrics concurrency-safe; adds/updates unit + integration coverage around close/cancel behavior.

Changes:

• Introduce reader-scoped context.Context cancellation; normalize post-close failures to net.ErrClosed and prevent new prefetch goroutines during shutdown.
• Simplify chunk coordination by removing per-chunk error state and tightening fetch/prefetch synchronization.
• Add unit/integration tests covering close cancellation of in-flight fetches/reads and related concurrency behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
audio/chunked-reader.go	Context-cancelable shutdown, fetch/prefetch coordination changes, latency metric locking/snapshotting
audio/chunked-reader_internal_test.go	Unit tests for Close() interactions with concurrent fetch, in-flight requests, and transport/body read behavior
audio/chunked-reader_integration_test.go	Integration tests refactor + new close-cancels-in-flight-read coverage

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