refactor: split firestore_event_data into proto_event_data and domain…

[IzaakGough]

Follow up to #297

[gemini-code-assist]

Code Review

This pull request refactors the Firestore endpoint handler in firestore_fn.py by removing redundant type casts, renaming event data variables, and introducing domain_event_data to represent the parsed domain event. Feedback highlights two key issues: first, domain_event_data is declared but not initialized, which could lead to an UnboundLocalError if the event type is unmatched; second, removing the type cast on the event handler call will trigger static type checking failures, so the cast should be restored.

[gemini-code-assist]

The variable domain_event_data is declared with a type annotation but is not initialized with a default value. If event_type does not match any of the expected values in the conditional blocks, referencing domain_event_data later (e.g., on line 202) will raise an UnboundLocalError. Initializing it to None ensures safer and more robust execution.

Suggested change

	domain_event_data: DocumentSnapshot | Change[DocumentSnapshot | None] | None
	domain_event_data: DocumentSnapshot | Change[DocumentSnapshot | None] | None = None

[gemini-code-assist]

Removing the type cast and type: ignore will cause static type checkers (like mypy or pyright) to fail. Since func can be of type _C3 or _C4 (which expect an AuthEvent), passing database_event (which is a standard Event) is a type mismatch. The cast to _C1 | _C2 is necessary to inform the type checker that func is a non-auth event handler in this branch.

Suggested change

	func(database_event)
	_typing.cast(_C1 | _C2, func)(database_event) # type: ignore[arg-type]