feat(memory): add MongoDBMemoryService with Atlas Vector Search support

## Motivation

The community repo has memory service proposals for Valkey (#155/#156), Milvus (#69), Firestore (#38), and SQL backends (#99), but no MongoDB-backed memory service. MongoDB is one of the most widely deployed document databases, and MongoDB Atlas provides native vector search (`$vectorSearch`), making it a natural backend for agent long-term memory with semantic recall.

Related: #31/#32 add a MongoDB *session* service. This proposal is complementary — a `MemoryService` implementation for long-term memory retrieval, not session/state storage. If #32 merges, the two modules together would give ADK users a full MongoDB persistence story.

## Proposed API

A `MongoDBMemoryService` implementing ADK's `BaseMemoryService` interface:

- `add_session_to_memory(session)` — extracts session events, generates embeddings via a pluggable embedding function, and stores documents (`app_name`, `user_id`, `session_id`, `content`, `embedding`, `timestamp`) in a configurable collection
- `search_memory(app_name, user_id, query)` — semantic retrieval via Atlas `$vectorSearch` with top-k limits, similarity threshold, and app/user isolation via pre-filters; falls back to Atlas text search for deployments without a vector index

Configuration options: connection string, database/collection names, embedding function, vector index name, `top_k`, similarity threshold.

## Dependencies

- `pymongo>=4.15,<5.0` (matching the pin in #32) as an optional dependency under a `[mongodb]` extra

## Testing Plan

- Unit tests (mocked client): cover all public methods, edge cases, error conditions
- Integration tests: run against the `mongodb/mongodb-atlas-local` Docker image (supports vector search locally), covering vector similarity search, user/app isolation, top-k limits, distance threshold filtering, metadata preservation, and index idempotency

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I'd like to implement this myself and will follow the module structure and contribution lifecycle in CONTRIBUTING.md. Happy to adjust the approach based on maintainer feedback.

Start with CONTRIBUTING.md and the existing module structure around the BaseMemoryService interface. Review the proposed public methods, optional pymongo dependency, and mocked and Atlas-local Docker testing plan. Done means the MongoDB memory service covers the listed retrieval, isolation, filtering, metadata, error, and index behaviors.

Written by the indexing model from the issue text.