fix: Auto-Cancellation of Assignments on Unavailability

[berat-sayin]

Related to #227

I have updated the availability service to ensure that when a volunteer marks themselves as "unavailable",
any active assignments are automatically cancelled and the associated help requests are returned to the
PENDING state.

Key Changes:

• backend/src/modules/availability/service.js:
  • Updated setAvailability to detect when isAvailable is set to false. If the volunteer has an active
    assignment, it is now cancelled via the repository, and the help request status is reset to
    PENDING.
  • Updated syncAvailability with similar logic: if the latest synced state is unavailable, any current
    assignment is cleared.
• Improved Integration Tests:
  • Added POST /api/availability/toggle to false cancels active assignment to verify that toggling
    availability to false deletes the assignment and resets the request.
  • Added POST /api/availability/sync to false cancels active assignment to verify the same behavior
    for bulk sync operations.
• Enhanced Unit Tests:
  • Added test cases to tests/unit/modules/availability/service.test.js for both setAvailability and
    syncAvailability to ensure they correctly interact with the repository when a volunteer becomes
    unavailable.

This ensures that requesters are not left waiting for a volunteer who is no longer active, and their
requests can be immediately picked up by other available volunteers.

[mehmetcangurbuz08]

Looks solid merging.