PYTHON-6019 Consolidate mongo_client module-level helpers into shared…

[NoahStapp]

… file

PYTHON-6019

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

Refactors MongoClient module-level helper logic by extracting common internals into a new shared module so the synchronous and asynchronous clients can reuse identical implementations.

Changes:

• Introduces pymongo/mongo_client_shared.py to host shared internal helpers (_add_retryable_write_error, _after_fork_child, _detect_external_db).
• Updates both pymongo/asynchronous/mongo_client.py and pymongo/synchronous/mongo_client.py to import and use the shared helpers instead of local duplicates.
• Adjusts register_at_fork wiring to call the shared _after_fork_child with the appropriate client registry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
pymongo/synchronous/mongo_client.py	Removes duplicated helper implementations and uses shared helpers; updates register_at_fork callback wiring.
pymongo/asynchronous/mongo_client.py	Removes duplicated helper implementations and uses shared helpers; updates register_at_fork callback wiring.
pymongo/mongo_client_shared.py	New internal module containing shared MongoClient helpers used by both sync/async implementations.

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[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pymongo/mongo_client_shared.py	86.36%	5 Missing and 1 partial ⚠️

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