fix(sync): keep the in-flight guard while a cancelled sync is running

[0xbrayo]

Part of #333.

SyncInterface.cancel() (called when WorkManager stops a SyncWorker) cleared the shared syncInFlight guard unconditionally. Interrupting the executor thread does not stop the native syncBoth call, so the cancelled sync kept running while the next sync (from the Handler chain, the alarm, or a manual tap) was allowed to start alongside it, with both writing to the same sync directory and database.

Change

• cancel() clears syncInFlight only when shutdownNow() returned a task that never started. Only then will that task's completion callback, which normally clears the guard, never run. That case was the original reason for clearing it here.
• A started task keeps the guard until its completion callback clears it, which happens once the native call returns.

Testing

• ./gradlew :mobile:testStandardDebugUnitTest passes.
• SyncInterface loads the native aw-sync library, which isn't available to JVM unit tests, so this has no new unit test.

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no actionable new issues remain.

Summary

This PR keeps syncInFlight set while a cancelled native sync is still running.

• cancel() releases the guard directly only when the executor removes an unstarted task.
• Rejected submissions now report cancellation through the completion callback.
• The previous submission-race finding is fixed. The thread is unnumbered, so it has no entry in previousFindings.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A["cancel()"] --> B["shutdownNow()"]
  B --> C{"Queued task removed?"}
  C -- Yes --> D["Clear syncInFlight"]
  C -- No --> E{"Submission rejected?"}
  E -- Yes --> F["Post cancellation callback"]
  E -- No --> G["Running task finishes"]
  G --> H["Post completion callback"]
  F --> D
  H --> D

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_{Reviews (2) · Last reviewed commit: "fix(sync): release the guard when a canc..." · Reviewed by Greptile}

[0xbrayo]

🤖 Claude, on behalf of @0xbrayo

@greptile review