GODRIVER-3648 Make server selection block on topology updates.

[matthewdale]

GODRIVER-3648

Summary

Remove the default case from the select statement in Topology.selectServerFromSubscription, making the loop block until there is a topology change.

Background & Motivation

A bug was introduced in #1515 that makes the Topology.selectServerFromSubscription loop non-blocking, resulting in a tight loop when there are no selectable servers.

[Copilot]

Pull Request Overview

This PR fixes a bug where server selection enters a tight loop when no selectable servers are available by removing the default case from a select statement to make topology updates blocking.

• Removes the default case from the select statement in Topology.selectServerFromSubscription
• Changes the server selection loop from non-blocking to blocking behavior
• Prevents CPU-intensive tight looping when waiting for topology changes

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[mongodb-drivers-pr-bot]

API Change Report

No changes found!

[mongodb-drivers-pr-bot]

🧪 Performance Results

Commit SHA: de38ea6

There were no significant changes to the performance to report for version 68dc7f808b51940007329b3e.

Benchmark	Measurement	% Change	Patch Value	Stable Region	H-Score	Z-Score

For a comprehensive view of all microbenchmark results for this PR's commit, please check out the Evergreen perf task for this patch.

[qingyang-hu]

Do we need a test case for the change?

[matthewdale]

Yeah, it would be a good idea to add a test. I'll try to find a sensible assertion we can make to check that it doesn't enter a busy wait.

[matthewdale]

Added a test that catches the busy-wait case.