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, _split_message, and _operation_to_command as module-level helpers in cursor_base.py
• 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 now accepts execute_fn(conn, op, read_preference) -> Response instead of routing through server.run_operation
• Remove Server.run_operation, Server.operation_to_command, and Server._split_message entirely

Note: _run_with_conn still passes address=conn.address and start=datetime.datetime.now() to run_cursor_command; these will be dropped as a trivial follow-up when PYTHON-5745 (#2891) merges.

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 new execute_fn 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). — PYTHON-5745 (PYTHON-5745 Consolidate command telemetry #2891) will clean up the address/start args once that PR merges.

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

This PR refactors cursor execution by moving the _Query/_GetMore command preparation + response construction logic out of Server.run_operation and into the cursor base classes (_AsyncCursorBase / sync equivalent). This continues the consolidation work so cursors execute via a cursor-owned execute_fn rather than routing through Server.

Changes:

• Removed Server.run_operation (and related helpers) in both sync/async server.py.
• Introduced cursor-base helpers (_CURSOR_DOC_FIELDS, _split_message, _operation_to_command) and added _run_with_conn as the new execution path for cursor operations.
• Updated MongoClient._run_operation (sync/async) to accept an execute_fn(conn, op, read_preference) -> Response, and updated cursor call sites + the stale getMore test accordingly.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/test_client.py	Updates stale getMore test to pass the new execute_fn callable (sync mirror).
test/asynchronous/test_client.py	Updates stale getMore test to pass the new execute_fn callable.
pymongo/synchronous/server.py	Removes cursor operation execution logic from Server.
pymongo/synchronous/mongo_client.py	Changes _run_operation to accept and invoke execute_fn instead of server.run_operation.
pymongo/synchronous/cursor.py	Switches cursor _send_message to route through _run_with_conn.
pymongo/synchronous/cursor_base.py	Adds _run_with_conn, module-level helpers, and an explicit abstract _unpack_response.
pymongo/synchronous/command_cursor.py	Switches command cursor _send_message to route through _run_with_conn.
pymongo/asynchronous/server.py	Removes cursor operation execution logic from Server.
pymongo/asynchronous/mongo_client.py	Changes _run_operation to accept and invoke execute_fn instead of server.run_operation.
pymongo/asynchronous/cursor.py	Switches cursor _send_message to route through _run_with_conn.
pymongo/asynchronous/cursor_base.py	Adds async _run_with_conn, module-level helpers, and an explicit abstract _unpack_response.
pymongo/asynchronous/command_cursor.py	Switches command cursor _send_message to route through _run_with_conn.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.23404% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pymongo/asynchronous/cursor_base.py	86.36%	3 Missing and 3 partials ⚠️
pymongo/synchronous/cursor_base.py	86.36%	3 Missing and 3 partials ⚠️

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