Modernize type hints: replace Optional[T] with T | None across codebase - #936

This updates type annotations to the Python 3.10+ union style by replacing Optional[...] with ... | None, aligning the codebase with current typing guidance. The change is mechanical and scoped to annotation/style consistency, with one related inline note updated to match the new syntax.

• Typing modernization (PEP 604)

  • Replaced Optional[T] annotations with T | None in core modules and tests.
  • Kept behavior unchanged; only annotation syntax was updated.

• Related cleanup

  • Removed obsolete typing.Optional imports left unused after conversion.
  • Removed stale typing imports (Type, List, Tuple) where no longer needed due to pyupgrade normalization.

• Documentation/comment alignment

  • Updated the inline comment in nipoppy/layout.py that described optional field typing so it now references OptionalFpathInfo | None instead of Optional[OptionalFpathInfo].

Example transformation applied throughout:

# before
from typing import Optional
def run(session_id: Optional[str] = None) -> None: ...

# after
def run(session_id: str | None = None) -> None: ...

• Fixes [MAINT] Convert Optional[<type>] to <type> | None #935

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