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Code Review

This pull request implements full-text search functionality for Firestore pipelines, adding a new search stage along with SearchOptions and supporting expressions like between, geo_distance, and document_matches. The changes include updates to the base pipeline, expression definitions, and stage logic, supported by new system and unit tests. Review feedback recommends expanding type hints for the between method to include integers and simplifying the Search stage's protobuf serialization by removing a redundant null check.