fix: propagate executor errors directly to the caller instead of emitting synthetic failure tasks

[JakubWorek]

Description

This PR aligns message/send error handling with other sdks by propagating the error instead of returning synthesized Task(FAILED). Streaming path is unchanged - synthesizing terminal failed status is correct there.

Fixes #370 🦕

[github-actions]

🧪 Code Coverage

⬇️ Download Full Report

	Base	PR	Delta
src/server/events/execution_event_queue.ts	94.44%	100%	🟢 +5.56%
src/server/request_handler/default_request_handler.ts	87.72%	88.46%	🟢 +0.74%
Total	90.65%	90.75%	🟢 +0.10%

Generated by coverage-comment.yml

[gemini-code-assist]

Code Review

This pull request refactors error handling in DefaultRequestHandler by propagating executor rejections directly up to the caller's Promise instead of publishing a synthetic failed task. This simplifies the error handling logic in _runExecutor and updates the test suite to assert that errors are thrown. The review feedback suggests making the new onExecutorError callback parameter required rather than optional to improve type safety and eliminate unnecessary conditional checks.

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