Add searchMeta stage tests

[danielfrankcom]

This change adds tests for the $searchMeta aggregation stage.

This is the first operator which uses the external mongot topology to provide a required search index. I defined a number of file-scoped pytest fixtures to define the necessary indexed collections for these tests, as an index of this type is far more expensive to set up than the ones we use elsewhere (on the order of 30-60 secs). Using a module scoped fixture allows us to amortize this cost across the test cases rather than taking the hit each time.

Note: The CI runs for this change will fail until #617 is merged, as it is heavily dependent. If you want to see the change in its correct (working) context, see the pr/searchMeta branch for now.

[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: 0.85 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (3308+7 LOC, 18 files); LLM: Adds new test coverage for the $searchMeta aggregation stage, including a new external topology fixture pattern; self-contained within the test-coverage component.

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