feat: make scheduled export cancellation cooperative and resumable

[p70436464-prog]

Closes#1184

PR Description:

Closes #1### Summary
This PR implements cooperative and resumable cancellation for scheduled exports, ensuring that operators have a bounded behavior and a safe recovery path when dependencies or workers fail.
### Changes made1.**Cooperative Cancellation (`AbortSignal` Integration):**- Added an `AbortController` instance to `ScheduledExportsWorker` to manage lifecycle signals.
- Passed the resulting `AbortSignal` down through `ScheduledExportsService.runDueSchedules` and `runSchedule`.
- Updated the `ObjectStorageClient` interface and its mock implementation (`HmacObjectStorageClient`) to accept and respect the `AbortSignal`. - This ensures that if the worker is stopped (or crashes gracefully), any in-flight S3 network I/O or ongoing processing halts cooperatively without lingering orphaned uploads.
2.**Resumable Idempotency (Durable Recovery without Duplicate Work):**- Modified the S3 object key timestamp suffix (`stamp`) for the generated `.csv` and `.json` artifacts inside `runSchedule`. - Previously, it used `now.toISOString()`, which meant a crashed worker would generate new redundant files with new timestamps when it restarted (hidden mutation/duplicate work). - Now, it strictly uses `schedule.nextRunAt.toISOString()`. If the worker restarts and processes the same due schedule, it deterministically overwrites the original, identically-named path. This strictly preserves durable state without duplicate work.
3.**Regression Test Coverage:**- Appended a new regression test suite: `worker handles cancellation cooperatively and preserves idempotency on restart`.
- The test deliberately mocks `ObjectStorageClient.uploadObject` to inject artificial delays, triggers an explicit `worker.stop()` interrupt, validates that the operation halts gracefully via `AbortError`, and then spins the worker back up to assert the exact same S3 destination key (`nextRunAt` stamp) is used on the recovery attempt.
### Acceptance Criteria Addressed-[x] Normal, degraded, and recovery states are explicit and bounded (abort sequences introduced).
-[x] Metrics and diagnostics are actionable, cardinality-bounded, and safe (reused paths, deterministic logging identifiers).
-[x] Recovery preserves durable state without duplicate work or hidden mutation (S3 destination keys bound to logical schedule time).
-[x] Failure-injection tests cover outage, restart, and recovery.

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