fix(sdk): use sync shutdown flush fallback

[abhinav-galileo]

Summary

• replace the atexit shutdown fallback that spawned a helper thread and asyncio.run() with a synchronous httpx.Client flush path
• reuse shared batch request construction for both async and sync send paths and clear the async client reference after fallback shutdown cleanup
• add regression coverage for the sync fallback path and for a short-lived subprocess that emits an SDK-evaluated observability event and exits without explicit shutdown

Testing

• make sync
• make check
• git push -u origin abhi/observability-sync-shutdown-flush (pre-push hook: lint, typecheck, contrib evaluator checks)

[codecov]

Codecov Report

❌ Patch coverage is 97.64706% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
sdks/python/src/agent_control/observability.py	97.64%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[namrataghadi-galileo]

nit: break instead of continue. It keeps the same behavior but is less awkward :D