fix(sdk): buffer async generator outputs in control decorator

[abhinav-galileo]

Summary

• add buffered async-generator support to @control() so decorated async generators stay protected
• normalize structured streamed chunks into JSON-safe post-check payloads instead of relying on lossy str(chunk)
• add SDK coverage for buffered replay, structured chunks, post-check blocking before replay, and stream failures

Behavior

• async generators are buffered before replay so post-stage deny/steer remains fail-closed
• this intentionally preserves safety semantics instead of real-time token streaming latency
• real-time streaming semantics remain follow-up work and should be tracked separately

Partial fix for #113

Testing

• uv run pytest sdks/python/tests/test_control_decorators.py
• make check

[codecov]

Codecov Report

❌ Patch coverage is 90.15544% with 19 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...dks/python/src/agent_control/control_decorators.py	90.05%	19 Missing ⚠️

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