PYTHON-6025 Consolidate non-IO code in AsyncClientSession/ClientSessi…

[sleepyStick]

…on and _Transaction into shared base classes

PYTHON-6025

Changes in this PR

Test Plan

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?

[codecov]

Codecov Report

❌ Patch coverage is 99.12664% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pymongo/client_session_shared.py	99.02%	0 Missing and 2 partials ⚠️

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[Copilot]

Copilot review overview

🟡 Changes recommended

Inherited public session members will be omitted from the generated API documentation unless inherited-member documentation is enabled.

Review effort: Balanced
Findings: 1

Open (1)

• Enable inherited members in session API documentation · New

What changed in this PR

Consolidates shared synchronous and asynchronous session/transaction logic into generic base classes.

Changes:

• Adds shared session and transaction base implementations.
• Retains I/O-specific context management and cleanup in concrete classes.
• Improves shared connection/server typing and removes obsolete type ignores.

File	Description
pymongo/​client_session_shared.py	Adds shared base implementations.
pymongo/​asynchronous/​client_session.py	Adopts shared bases for async sessions.
pymongo/​synchronous/​client_session.py	Adopts shared bases for sync sessions.
pymongo/​typings.py	Adds an agnostic server type.
pymongo/​message.py	Removes unnecessary type ignores.

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[blink1073]

@sleepyStick did you verify this one?

[sleepyStick]

hmm okay maintaining docs in the concrete classes sounds kinda annoying to maintain cuz the logic and the docs would live in two different places so i've opted for using :inherited-members: but lmk if that's the wrong choice

[blink1073]

Yeah the rendered docs look good now!

[blink1073]

LGTM