fix(core): cancel pending tasks in abatch_as_completed on early exit or exception

[GAUTAM V DATLA (gautamvarmadatla)]

Fixes: #35419

abatch_as_completed now creates explicit asyncio.Task objects and ensures they are cleaned up on exit. The iteration is wrapped in a try/finally so any unfinished tasks are cancelled and all tasks are awaited with gather(..., return_exceptions=True). Waiting on all tasks also prevents "Task exception was never retrieved" warnings if the caller stops iterating while multiple tasks finish at the same time. I also added two unit tests to cover the exception and early-break paths.

[codspeed-hq]

Merging this PR will improve performance by 25.71%

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⚠️ Different runtime environments detected

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⚡ 1 improved benchmark
✅ 12 untouched benchmarks
⏩ 27 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	test_async_callbacks_in_sync	19.3 ms	15.4 ms	+25.71%

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_{Comparing gautamvarmadatla:fix/abatch-as-completed-task-leak (befb4f5) with master (b3dff4a)}

Footnotes

1. 27 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩