PYTHON-5856 Move cursor execution logic into cursor class

[aclark4life]

PYTHON-5856

Changes in this PR

Moves the command preparation and response construction that lived in Server.run_operation into _AsyncCursorBase._run_with_conn, completing the consolidation started in PYTHON-5676. Cursors now call run_cursor_command directly without routing through Server.

• Extract _CURSOR_DOC_FIELDS and _split_message (in cursor_shared.py) and _operation_to_command (in cursor_base.py) as module-level helpers
• Add abstract _unpack_response to _AsyncCursorBase to make the interface explicit
• Add _run_with_conn (carrying the @_handle_reauth decorator) to _AsyncCursorBase — new home for all of Server.run_operation's logic
• client._run_operation's second parameter, previously unpack_res (only decoded the wire response), is replaced by run_with_conn(conn, operation, read_preference) -> Response, which executes the whole operation instead of routing through server.run_operation; callers (cursor.py, command_cursor.py) pass self._run_with_conn
• Remove Server.run_operation, Server.operation_to_command, and Server._split_message entirely

PYTHON-5745 (#2891) merged into master and was picked up here via a merge from master. That merge left Server.run_operation/operation_to_command (and their now-orphaned imports) unreconciled since master no longer had the caller this branch had moved to cursor_base.py; this PR removes that leftover code and drops the address/start args from _run_with_conn's call to run_cursor_command, matching the new _CommandTelemetry-based signature.

Test Plan

No new tests — this is a behavior-preserving refactor. The existing spec suites (APM/command-monitoring, auth, cursor) assert the exact behavior that any regression would change. Updated test_stale_getmore to use the run_with_conn parameter name.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)? — Not necessary; internal refactor with no user-facing behavior change.
• Is there test coverage? — Covered by existing spec suites.
• Is any followup work tracked in a JIRA ticket? If so, add link(s). — N/A

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?

[Copilot]

Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated no new comments.