Fix/HYBIM-962 review follow ups

[pradystar]

Summary

Harden OTLP content mapping, acknowledgement diagnostics, and operation trace ownership for several edge cases found during review.

Follow ups from reviews

1. https://github.com/splunk/splunk-ao-python/pull/191#pullrequestreview-4824427206 check the minor(bug) in this review.
2. fix(otel): surface successful-response span rejections #188 (review) check the follow up in this review.
3. Fix/SAO-259 operation trace ownership #187 (review) follow up and minor review

What changed

• Preserve explicitly empty message parts lists instead of serializing them as the text "[]".
• Recognize finite, positive integral JSON-number rejection counts such as 3.0 in OTLP partial-success acknowledgements.
• Centralize current-root ownership checks shared by decorators, synchronous/asynchronous handlers, and OpenAI Agents failure cleanup.
• Add regression coverage for invalid rejection counts and async decorated-operation exception cleanup.
• Document the user-visible fixes in the changelog.

Testing

• poetry run pytest — 2,157 passed, 4 skipped.
• Focused converter, exporter, decorator, handler, and OpenAI Agents suites — 446 passed, 1 skipped.
• inv type-check — passed for 116 source files.
• Changed-file pre-commit hooks, poetry check --lock, and git diff --check passed.