Change Scheduler.delete() to return bool or raise instead of bool | Exception

Scheduler.delete() and AsyncScheduler.delete() currently return bool | Exception — the exception is returned as a value rather than raised, so callers can't handle failures idiomatically and type checkers can't help.

Proposal: return bool (True if deleted, False if the job didn't exist) and raise on unexpected errors, or raise a specific library exception instead of the current blanket except Exception.

This is a breaking API change, so it should land with the next breaking-version bump. Deferred from #117 to keep that PR behavior-compatible. Related TODOs already exist in fastapi_gcp_tasks/scheduler.py and async_scheduler.py.

Start by reading the TODOs in fastapi_gcp_tasks/scheduler.py and async_scheduler.py and comparing both delete() implementations. Resolve the return and exception contract, including whether a specific library exception is needed, then ensure both methods follow it and that the change is scheduled for the next breaking-version bump.