Add $connPoolStats tests

[PatersonProjects]

This PR adds the tests to validate the $connPoolStats diagnostic command.

Ref: Issue #43

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort L · Status Needs Review
Confidence: 0.75 (mixed)

Reasoning

component from path globs (test-coverage); effort from diff stats (224+0 LOC, 6 files); LLM: Adds new test cases for the $connPoolStats diagnostic command, expanding compatibility test coverage for a tracked feature.

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