fix: avoid retrying aborted LLM requests during shutdown

[tobiplancraft]

Summary

• treat already-aborted inference LLM streams as expected cancellation after request creation
• return early for aborted streams in the error path so shutdown does not reclassify cancellation as a retryable API failure
• prevent noisy retry logs when a participant disconnect aborts an in-flight goodbye generation

Test plan

• Reproduce the bug locally with a modified manual_shutdown.ts end-call flow
• Build @livekit/agents
• Verify the fix removes retry spam for aborted LLM requests during session shutdown

Fixes #1245

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[devin-ai-integration]

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