Fix/371 repeat every task lifecycle

[Binilkks]

Summary

This PR addresses issue #371 by improving lifecycle management for repeated background tasks created by repeat_every.

Previously, repeat_every started tasks in the background without exposing a task handle, which made it hard to inspect or cancel running tasks during shutdown/reload scenarios.

What Changed

• Switched task creation from ensure_future to create_task in fastapi_utils/tasks.py.
• Added internal tracking of active repeated tasks using a weak-reference registry in fastapi_utils/tasks.py.
• Added new helper API:
  • get_repeated_task(task_func): returns the active task handle (or None)
  • cancel_repeated_task(task_func): cancels active task and returns success status
• Added done-callback cleanup so completed tasks are removed from the internal registry.
• Added tests for sync and async paths to verify task handle visibility and cancellation behavior in tests/test_tasks.py.

Why

This provides a safer lifecycle path for long-running services:

• Inspect running repeated tasks
• Cancel repeated tasks explicitly
• Reduce orphan/background task management risk

Behavior Impact

• Existing repeat_every usage remains compatible.
• New lifecycle helpers are additive and optional.
• Task execution behavior (seconds, wait_first, max_repetitions, on_exception, on_complete) is unchanged.

Testing

• Added new unit tests in tests/test_tasks.py for:
  • task handle retrieval
  • task cancellation (sync and async function variants)
• Note: local test execution in current environment reported missing async pytest plugin setup, so full runtime verification should be done in CI/project test environment.

Related

Closes #371