test(source/firestore): migrate integration tests to MCP endpoint

[DylanVolz]

Description

Migrates the Firestore integration tests from the legacy /api endpoint to the MCP endpoint (/mcp), following #2922. The legacy REST test is kept, and both transports share setup and assertions.

• Adds firestore_mcp_test.go: MCP tools/list for all 13 tools, the existing JSON-RPC error-path checks, and every tool invocation check over MCP.
• Refactors the legacy file into shared setup (setupFirestoreTest) and a small REST/MCP transport; request bodies are now args maps.
• Teardown is registered with t.Cleanup before seeding and uses a bounded context.WithoutCancel context; test documents are seeded in one BulkWriter batch and the client is closed on cleanup.

Testing: Ran both tests locally against the Firestore emulator with dummy credentials: tools/list and 100 subtests pass on both transports. analyzeQuery and the get/validate rules tools need real Firestore and the Firebase Rules API, so those are verified by the CI firestore job.

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🛠️ Fixes #<issue_number_goes_here>

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[gemini-code-assist]

Code Review

This pull request refactors the Firestore integration tests to support both REST and MCP transports by introducing a firestoreTransport helper and a firestoreTestData struct. It also optimizes test data setup and cleanup using Firestore's BulkWriter and t.Cleanup, and adds a new test file firestore_mcp_test.go to validate MCP tool listings and invocations. The review feedback correctly identifies a logical gap in the invokeCase helper where tests could silently pass if a tool invocation unexpectedly succeeds when an error was expected, and provides a clear code suggestion to address this issue.