fix(mongodb): add MongoDBAtlasLocalContainer for Atlas Search and Vector Search testing

[caseyclements]

New Container

Fixes MongoDB JIRA ticket INTPYTHON-922

Summary

Adds MongoDBAtlasLocalContainer to the existing mongodb module, enabling local testing of MongoDB Atlas-specific features such as Atlas Search and Atlas Vector Search.

The container uses the mongodb/mongodb-atlas-local Docker image, which provides a fully functional local MongoDB Atlas deployment including a single-node replica set and the MongoT search indexing service.

What's included

• MongoDBAtlasLocalContainer class in modules/mongodb/testcontainers/mongodb/__init__.py
  • Uses ExecWaitStrategy(["runner", "healthcheck"]) for reliable startup detection
  • get_connection_string() returns a mongodb://host:port/?directConnection=true URI
  • get_connection_client() returns a connected pymongo.MongoClient
• Tests in modules/mongodb/tests/test_mongodb.py
  • Basic CRUD test (test_docker_run_mongodb_atlas_local)
  • Connection string format test (test_mongodb_atlas_local_connection_string)
  • Vector search integration test (test_vector_search) — creates a vector index, waits for indexing, and queries with $vectorSearch
• README.rst updated with MongoDBAtlasLocalContainer autoclass directive

PR Checklist

• Your PR title follows the Conventional Commits syntax
  • Suggested title: fix(mongodb): add MongoDBAtlasLocalContainer for Atlas Search and Vector Search testing
• Your PR allows maintainers to edit your branch, this will speed up resolving minor issues!
• The new container is implemented under modules/*
  • Added to existing modules/mongodb module (not a new module)
  • Package namespacing follows testcontainers.mongodb.*
  • No __init__.py above module level
  • Tests under modules/mongodb/tests
  • README.rst updated
• Your module is added in pyproject.toml
  • Already declared — this extends the existing mongodb module
  • No new dependencies — uses existing pymongo>=4 optional dependency

Preferred implementation

This implementation follows the maintainer preference of minimal dependencies:

• No additional client libraries — uses pymongo which is already an optional dependency of the mongodb module
• Startup wait strategy — uses ExecWaitStrategy(["runner", "healthcheck"]) (matching the health check approach used by testcontainers-java's Atlas module) rather than log-based detection
• The vector search test uses a simple hashlib-based mock embedding function — no numpy or ML library dependencies