PYTHON-3900 Add a check for PR diff coverage

[blink1073]

PYTHON-3900

Changes in this PR

Enable Codecov's patch coverage status check so pull requests are gated on test coverage of newly added/modified lines (80% target).

Test Plan

Validated the updated .codecov.yml against Codecov's config validation endpoint (returned "Valid!").

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). PYTHON-5949 to track follow up work to add GitHub status checks.

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 updates the repository’s Codecov configuration to enforce patch (diff) coverage targets on pull requests, helping ensure newly added/modified lines are adequately covered by tests.

Changes:

• Enables Codecov patch coverage status reporting.
• Sets a patch coverage target of 80% with 0% threshold (no allowed drop below target).