fix(mongodb): Use wait strategy instead of deprecated wait_for_logs

Summary

Replace the deprecated wait_for_logs function with LogMessageWaitStrategy in the MongoDB container module.

This is part of a fix for #874 and similar to:

• fix(minio): Use wait strategy instead of deprecated decorator #899 (MinIO - merged)
• fix(elasticsearch): Use wait strategy instead of deprecated decorator #915 (Elasticsearch - merged)
• feat: Add ExecWaitStrategy and migrate Postgres from deprecated decorator #935 (Postgres - merged)
• fix(kafka): Use wait strategy instead of deprecated wait_for_logs #903 (Kafka/Redpanda - open)
• fix(redis): Use wait strategy instead of deprecated decorator #914 (Redis - open)
• fix(redis): Use wait strategy instead of deprecated decorator #941 (Redis - open)

Changes

• Import LogMessageWaitStrategy instead of wait_for_logs
• Pass _wait_strategy=LogMessageWaitStrategy(...) to parent constructor
• Override _connect() with empty implementation (DbContainer's base implementation uses SQLAlchemy which doesn't apply to MongoDB)

Testing

All existing MongoDB tests pass:

poetry run pytest modules/mongodb/tests/ -v
======================== 4 passed in 16.26s ========================

No deprecation warnings from testcontainers after this change.