Implements aclose() and cancel() methods on CrewStreamingOutput and
FlowStreamingOutput to allow callers to abort in-flight streaming and
release resources promptly (e.g. when an HTTP client disconnects).
- Add aclose() async method for async streaming contexts
- Add cancel() sync method for sync streaming contexts
- Add is_cancelled property to check cancellation state
- Update create_async_chunk_generator to support cancel events
- Update create_chunk_generator to support cancel thread events
- Add comprehensive tests for cancellation scenarios
Closes#5312
Co-Authored-By: João <joao@crewai.com>
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