fix(scheduler): honor backfill overlap policy

[officialasishkumar]

What changed?

Backfilled schedule fires now use the overlap policy from the queued backfill request when it is set. Unset backfill policies continue to fall back to the schedule-level policy. Closes #8005.

Why?

BackfillScheduleRequest and BackfillSignal preserve a request-specific OverlapPolicy, but processBackfills previously called processScheduleFire, which always built the fire activity request with input.Policies.OverlapPolicy. As a result, a one-off backfill policy override was ignored at execution time.

The fix routes backfilled fires through an overlap-policy-aware helper and keeps the default scheduled-fire path unchanged.

How did you test it?

go test ./service/worker/scheduler -run 'TestSchedulerWorkflowBackfillUsesBackfillOverlapPolicy|TestBackfillOverlapPolicy|TestProcessBackfillsRespectsPause|TestBackfillFireComputation'\n\ngo test ./service/worker/scheduler\n\nmake pr GEN_DIR=service/worker/scheduler\n\nPotential risks\n\nLow. This only changes backfill execution when the backfill request supplies an explicit overlap policy. Backfills without an explicit policy retain the prior schedule-policy behavior.\n\nRelease notes\n\nBackfill schedule requests now honor their requested overlap policy when firing backfilled runs. #8005\n\nDocumentation Changes\n\nN/A\n

[gitar-bot]

Code Review ✅ Approved

Updates the scheduler to correctly enforce the backfill overlap policy. No issues found.

Rules ✅ All requirements met

Repository Rules

✅ GitHub Issue Linking Requirement: The PR description contains 'Closes #8005', which correctly references a valid cadence-workflow GitHub issue.

✅ PR Description Quality Standards: The PR description provides a clear 'What changed?', 'Why?', test commands, identifies low risks, includes release notes with an issue link, and correctly notes documentation changes.

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