bugfix: defer stream cancellation to scheduler boundary.

[phantomlei3]

Description

Issue: In PD-disaggregated serving, a failed Decode streaming response could cancel a request
asynchronously between the scheduler’s terminal-state scan and running-queue rebuild, violating
scheduler invariants and triggering a fatal error that terminated the Decode process.

Fix: Defer streaming cancellations through a thread-safe queue and apply them at the next scheduler
boundary, ensuring safe state transitions and KV Cache release. The fix supports single, batched,
duplicate, and late cancellations, with corresponding unit-test coverage.

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactor
• Documentation
• Test
• Build or CI

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