[GoldenEmperor1177]

PYTHON-6145

Changes in this PR

The let argument of Cursor and AsyncCursor was annotated as Optional[bool], but the code requires a document (validate_is_document_type), the same as Collection.find. A document was a mypy error, and a bool, which raises TypeError at runtime, was accepted. The annotation is now Optional[Mapping[str, Any]]. It is one line in pymongo/cursor_shared.py, shared by both cursor classes, so there is no runtime change. Added a changelog entry and a type checking test.

AI disclosure: I used an AI assistant (Claude Code) to help find the cause, write the fix and the tests. I read the change and I understand it.

Test Plan

Added test_cursor_let (an only_type_check test) to test/test_typing.py. With the old annotation, mypy 1.19.1 and pyright 1.1.411 both report an error for it, and with the new one both pass on test_typing.py and test_typing_strict.py. mypy over bson, gridfs, tools and pymongo gives the same 41 errors with and without the change, all from optional packages missing in my environment. Against a local MongoDB 7.0, test_typing, test_cursor, test_collection, test_objectid and the let and cursor tests in test_crud_unified (sync and async) pass. sphinx -W, ruff, ruff format and the pre-commit hooks pass.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?