PYTHON-6018 Consolidate backpressure helpers into helpers_shared.py

[NoahStapp]

PYTHON-6018

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 consolidates the client backpressure retry/backoff helper (_RetryPolicy + exponential backoff) into pymongo/helpers_shared.py so both synchronous and asynchronous clients (and their tests) reference the same implementation.

Changes:

• Moved _RetryPolicy and _backoff implementation from pymongo/{a,}synchronous/helpers.py into pymongo/helpers_shared.py.
• Updated both sync/async MongoClient implementations to instantiate helpers_shared._RetryPolicy.
• Simplified retryable read/write tests to patch pymongo.helpers_shared._RetryPolicy.backoff directly (removing _IS_SYNC branching).

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/test_retryable_writes.py	Updates backoff mocking target to the shared helper module.
test/test_retryable_reads.py	Updates backoff mocking target to the shared helper module.
test/asynchronous/test_retryable_writes.py	Updates backoff mocking target to the shared helper module for async tests.
test/asynchronous/test_retryable_reads.py	Updates backoff mocking target to the shared helper module for async tests.
pymongo/synchronous/mongo_client.py	Switches sync client retry policy construction to helpers_shared._RetryPolicy.
pymongo/synchronous/helpers.py	Removes local backpressure helper implementation now housed in helpers_shared.
pymongo/helpers_shared.py	Adds shared backpressure helpers (_backoff, _RetryPolicy) and CSOT-aware should_retry.
pymongo/asynchronous/mongo_client.py	Switches async client retry policy construction to helpers_shared._RetryPolicy and updates should_retry call to be non-awaited.
pymongo/asynchronous/helpers.py	Removes local backpressure helper implementation now housed in helpers_shared.

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[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pymongo/helpers_shared.py	95.65%	0 Missing and 1 partial ⚠️

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[blink1073]

LGTM