Add $geoNear stage tests

[danielfrankcom]

This change adds tests for the $geoNear aggregation stage operator.

As part of this change I introduced an indexes property to StageTestCase since I have needed this for a few operators in this category now.

I also introduced a foreign_indexes property to LookupTestCase, and defined the integration test for $lookup and $geoNear on the existing test_stages_position_lookup.py file, since it has the necessary infrastructure to deal with foreign documents already. This avoided complicating the $geoNear integration test file.

Ref: #34

[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 (7507+1 LOC, 23 files); LLM: Adds new test coverage for the $geoNear aggregation stage, extending existing test infrastructure with new indexes and foreign_indexes properties across test case types.

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