PYTHON-5517 Updates to connection pool backoff behavior and tests

[blink1073]

PYTHON-5517

Summary

Update the behavior of connection pool backoff to match the technical design doc and add/update tests.

Changes in this PR

Behavior Changes

• Network timeouts should also induce backoff state.
• An operation that is blocked waiting for backoff in the connection checkout process SHOULD be unblocked when an in-use connection is returned to the pool. This avoids forcing one operation to pay the latency cost of waiting for backoff when other connections become available.
• Backoff messages should include the attempt number
• After backoff, the pool should return to ready state
• Successful heartbeat should not mark the pool as ready when in backoff mode

Test Changes

• Update the tests that used a network timeout with a hello - return another error type and don't block or close the connection.
• Verify that a connection close on a hello does not clear the pool
• Verify that a connection timeout on a hello does not clear the pool
• Verify that the pool returns to ready state after backoff
• Verify the pool backoff monitor events content
• Verify the pool backoff log message content
• Verify that we can transition from backoff to error state
• Verify that a failed heartbeat clears the pool if in backoff state
• Verify that a successful heartbeat does not change the pool to ready if in backoff

Testing Plan

• Run the connection_logging, connection_monitoring, load_balancing, and SDAM tests locally
• Tests are run in Evergreen automatically in PRs

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?

Checklist for Reviewer {@primary_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?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?