PYTHON-6014 Move internal pool classes into pool_shared.py

[NoahStapp]

PYTHON-6014

Changes in this PR

See ticket.

Test Plan

Existing test suite.

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?

[Copilot]

Pull request overview

This PR refactors PyMongo’s connection pool implementation by relocating a few shared internal pool helpers (generation/state tracking and closed-pool error) into pymongo/pool_shared.py, and updates imports across synchronous/asynchronous pool code and the related test utilities.

Changes:

• Moved _PoolClosedError, _PoolGeneration, and PoolState definitions into pymongo/pool_shared.py.
• Updated pymongo.synchronous.pool and pymongo.asynchronous.pool to import those shared helpers from pool_shared.
• Updated tests/utilities to import the moved helpers from pymongo.pool_shared instead of the per-implementation pool modules.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pymongo/pool_shared.py	Adds shared internal pool classes (_PoolClosedError, _PoolGeneration, PoolState) so both pool implementations can reuse them.
pymongo/synchronous/pool.py	Removes local definitions and imports the shared pool helpers from pool_shared.
pymongo/asynchronous/pool.py	Removes local definitions and imports the shared pool helpers from pool_shared.
test/utils.py	Switches sync test utilities to import _CancellationContext / _PoolGeneration from pool_shared.
test/utils_shared.py	Switches shared test utilities to import _CancellationContext / _PoolGeneration from pool_shared.
test/test_transactions.py	Updates sync transactions tests to import PoolState from pool_shared.
test/test_connection_monitoring.py	Updates sync CMAP tests to import PoolState / _PoolClosedError from pool_shared.
test/asynchronous/utils.py	Switches async test utilities to import _CancellationContext / _PoolGeneration from pool_shared.
test/asynchronous/test_transactions.py	Updates async transactions tests to import PoolState from pool_shared.
test/asynchronous/test_connection_monitoring.py	Updates async CMAP tests to import PoolState / _PoolClosedError from pool_shared.

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[aclark4life]

nice!