[MCP] firestore_query_collection does not seem to be working for sub collections

## Summary

`firestore_query_collection` does not seem to be working for sub collections

## Bug Info

**Affected Tool(s):** Firebase MCP tools, `firestore_query_collection` - https://github.com/firebase/firebase-tools/blob/main/src/mcp/tools/firestore/query_collection.ts
**MCP Client:** Claude 1.0.2339 (1782e2) 2025-12-16T19:35:52.000Z
**Operating System:** macOS Tahoe 26.2

### Steps to Reproduce

- Set up the MCP server and connect Claude Desktop to it
- Start asking Claude about the Firestore database project, it can access most of the data but not sub collections
- Ask it to query a sub collection e.g. "tell about about all of the transactions for each trip"
- Claude gets an error: Collection id "trips/0NK4xpv3AMbbdUq9ypIc/transactions" is invalid because it contains "/"

More detail from Claude:

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**Key part:** The `collection_path` description says:
> "A collection path (e.g. `collectionName/` or `parentCollection/parentDocument/collectionName`)"

This **explicitly states** that subcollection paths like `parentCollection/parentDocument/collectionName` should work!

**But when I try it, I get:**
> Collection id "trips/0NK4xpv3AMbbdUq9ypIc/transactions" is invalid because it contains "/"

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So transactions is a collection within each trip document.

It would be amazing to build an AI application using Firestore and the MCP tools however this seems like a blocker. Unless I'm missing something?

Here is my schema: