Add $collStats aggregation stage tests

[danielfrankcom]

This change adds tests for the $collStats operator.

Due to the nature of the operator, I needed to take a different approach to the assertions here. We can't reliably capture the entire expected shape of the result, so I added some machinery for asserting properties of the result. This should let us provide the coverage we need for $collStats, but also be applicable to future operators with complex results structures.

I also tried to avoid implementation-specific logic, since these stats seem to expose different things depending on the engine internals. The tests here are focused on the operator spec as best as possible.

I had to move strict_equal to its own module to avoid a circular import issue. The content is exactly the same, I just pulled it out.

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort XL · Status Needs Review
Confidence: 0.80 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (2270+53 LOC, 19 files); LLM: Adds new test coverage for the $collStats aggregation stage, including new assertion machinery for complex result structures; touches test files and result-analyzer logic but is scoped to the test-coverage component.

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