Abort HTTP/3 upload tasks on drop

[ryanfowler]

Summary

• Wrap HTTP/3 upload tasks in an abort-on-drop guard as soon as they are spawned.
• Preserve the finished-task fast path while ensuring in-flight uploads do not outlive request cancellation or response errors.
• Add a regression test covering drop behavior for the upload task wrapper.

Testing

• Added a unit test for H3UploadTask drop behavior.
• Formatting and linting were run on the implementation path; targeted validation passed.