feat(integrations): add MongoDB toolset with vector and hybrid search

[theshanbhag]

Add an experimental MongoDbToolset under google.adk.integrations.mongodb that lets agents run Atlas Vector Search ($vectorSearch) and hybrid full-text + vector search ($rankFusion, reciprocal rank fusion) against a MongoDB database. Search results exclude raw embedding vectors by default and include a search_score; limits and defaults are controlled via MongoDbToolSettings.

Usage:
toolset = MongoDbToolset(
connection_string="mongodb+srv://...",
database_name="my_db",
)
agent = Agent(model="gemini-2.5-flash", tools=[toolset])

Requires the pymongo package: pip install google-adk[mongodb].

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