chore: clean cancelled read requests earlier in driver state transitions

[caicancai]

Handle cancelled reads earlier in on_event (Request/Polled) and keep requests_by_offset in sync when cancellation is detected before poll promotion.

Purpose
When read futures are dropped quickly around registration/poll transition, cancelled requests no longer linger in internal state. This reduces stale index entries, avoids repeated coalescing scans over requests that will never run, and keeps queue/index state more consistent without changing existing dequeue/coalescing correctness checks.

[codspeed-hq]

Merging this PR will not alter performance

✅ 1135 untouched benchmarks
⏩ 1268 skipped benchmarks¹

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_{Comparing caicancai:fix/read-early-cancel-cleanup (c771915) with develop (6b5a92d)}

Footnotes

1. 1268 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. ↩

[robert3005]

Apologies for the delay, since we made some CI changes you will need to rebase to get this merged

[caicancai]

Apologies for the delay, since we made some CI changes you will need to rebase to get this merged

@robert3005 Thanks, I performed a merge; will that solve the problem?