PYTHON-5716 - Clarify expected error if backoff exceeds CSOT's deadli…

[NoahStapp]

…ne in withTransaction

PYTHON-5716

Changes in this PR

Second implementation of DRIVERS-3391.

Test Plan

Added a new test test_callback_not_retried_after_csot_timeout to verify the new behavior, as Python does not implement the withTransaction CSOT unified tests.

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?

[sleepyStick]

LGTM (pending passing tests, but the code changes make sense to me)

[NoahStapp]

Encryption test failures are expected until we rebase backpressure to master.