fix: type hint when whatchfiles library is not installed

[danfimov]

When taskiq scheduler --help called without watchdog installed, this issue occur:

Traceback (most recent call last):
  File "/home/s3rius/projects/github/taskiq/taskiq/.venv/bin/taskiq", line 10, in <module>
    sys.exit(main())
             ~~~~^^
  File "/home/s3rius/projects/github/taskiq/taskiq/taskiq/__main__.py", line 47, in main
    cmd_class = entrypoint.load()
  File "/home/s3rius/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/importlib/metadata/__init__.py", line 179, in load
    module = import_module(match.group('module'))
  File "/home/s3rius/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/importlib/__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 1026, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/home/s3rius/projects/github/taskiq/taskiq/taskiq/cli/worker/cmd.py", line 6, in <module>
    from taskiq.cli.worker.run import run_worker
  File "/home/s3rius/projects/github/taskiq/taskiq/taskiq/cli/worker/run.py", line 15, in <module>
    from taskiq.cli.worker.process_manager import ProcessManager
  File "/home/s3rius/projects/github/taskiq/taskiq/taskiq/cli/worker/process_manager.py", line 149, in <module>
    class ProcessManager:
    ...<139 lines>...
                        )
  File "/home/s3rius/projects/github/taskiq/taskiq/taskiq/cli/worker/process_manager.py", line 162, in ProcessManager
    observer: Observer | None = None,  # type: ignore[valid-type]
              ~~~~~~~~~^~~~~~
TypeError: unsupported operand type(s) for |: 'NoneType' and 'NoneType'

This pull request fixes it by using old-style type hint Optional

[Copilot]

Pull request overview

This PR addresses a type-checking issue by changing the type hint for the observer parameter from the modern PEP 604 union syntax (Observer | None) to the old-style Optional[Observer]. This change is specifically made for a conditionally defined type (Observer) that can be None when the optional watchdog dependency is not installed, which appears to cause issues with type checkers.

• Changed observer parameter type hint to use Optional[Observer] instead of Observer | None
• Added Optional import from typing module
• Added # noqa: UP045 to suppress ruff's modernization suggestion

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