Attempt cancellation of pending futures and cap workers by file count in GCSTimeSpanFileTransformOperator

[SameerMesiah97]

Description

This change is a follow-up to PR #62196, which introduced parallel download and upload support in GCSTimeSpanFileTransformOperator. It adds best-effort cancellation of pending futures when a GoogleCloudError occurs during parallel execution and caps the number of worker threads to the number of files being processed. When *_continue_on_fail=False, the operator still raises on the first failure, but now also calls cancel() on submitted futures to prevent queued tasks from starting, reducing unnecessary work in systemic failure scenarios.

Rationale

Previously, the operator raised on failure but allowed all queued tasks to proceed, and could create more worker threads than necessary for small batches. In high-volume workloads, this could result in unnecessary GCS API calls after a failure was already known, while in low-volume workloads it led to inefficient thread usage. Cancelling pending futures reduces wasted work and capping workers ensures more efficient resource utilisation, while preserving existing behaviour and keeping failure semantics unchanged.

Tests

• Updated existing parallel execution tests to assert that worker count is capped to the number of files and that cancel() is invoked when *_continue_on_fail=False (and not invoked when *_continue_on_fail=True).
• Extended existing failure-path tests for both download and upload flows using controlled executor and as_completed mocking; system tests are not required as external service interaction remains unchanged.

Backwards Compatibility

No change in behaviour; task failure semantics remain unchanged.

System Tests

The system test example_gcs_transform_timespan.py has been run and has passed succesfully after implementing thse changes. Due to OpenLineage compatibility issues, the check_openlineage_events task had to be disabled but the current test results are sufficient to indicate that the new functionality is working as intended with no regressions.The screenshot for these results has been provided below:

[VladaZakharova]

hi there, thanks for the PR!
Can you please upload the screenshot of green system tests?

[SameerMesiah97]

@VladaZakharova

I have added the system test results to the PR description.