feat(integrations): add MongoDB-backed session and memory services

[theshanbhag]

Add MongoDbSessionService, which persists sessions, events, and app:/user:-scoped state in MongoDB with optimistic-concurrency revisions, and MongoDbMemoryService, which ingests session events into a keyword-indexed memory collection for long-term recall. Register the MONGODB_* experimental features (enabled by default) and export both services from google.adk.integrations.mongodb.

Also add a README documenting the MongoDB integration (search toolset, both services, and MongoDbToolSettings) and add mongomock to the test dependencies for the new unit tests.

NO_UNIT_GUIDE=The integration README added in this commit documents all MongoDB units; per-unit guides will follow when the experimental API stabilizes.

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