Add search stage tests

[danielfrankcom]

This change adds tests for the search aggregation stage.

The mongot sidecar change here overlaps with #638. Both contain the same commit that should merge cleanly regardless of which is merged first.

I defined a number of 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/package scoped fixture allows us to amortize this cost across the test cases rather than taking the hit each time.

Closes #374

[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: 0.30 (deterministic)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (9360+6 LOC, 40 files); LLM failed: Invalid response body while trying to fetch https://api.anthropic.com/v1/messages: Premature close

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[tonglyu]

returnScope and searchNodePreference options are missing in the test