test(source/cloud-storage): migrate integration tests to MCP endpoint

[DylanVolz]

Description

Migrates the Cloud Storage 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 cloud_storage_mcp_test.go: MCP tools/list for all 27 tools (including the configured variants, whose baked-in parameters are dropped from the schema) and every tool invocation check over MCP.
• Refactors the legacy file into shared setup (setupCloudStorageTest) and a REST/MCP invokeTool; request bodies are now args maps.
• Bucket cleanup is registered with t.Cleanup before the bucket is created and uses a bounded context.WithoutCancel context; seed objects are written from one table and the client is closed on cleanup.

Testing: Ran both tests locally against fake-gcs-server: tools/list and 187 subtests pass on both transports. Bucket IAM policy, objects.move, and bucket location/uniform-access settings aren't supported by the emulator, so those are verified by the CI cloud-storage job.

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🛠️ Fixes #<issue_number_goes_here>

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[gemini-code-assist]

Code Review

This pull request refactors the Cloud Storage integration tests to support both REST and Model Context Protocol (MCP) transports. It introduces a cloudStorageTransport abstraction to handle starting the server and invoking tools over both protocols, and migrates test payloads from raw JSON strings to structured maps. Additionally, a new test file cloud_storage_mcp_test.go is added to explicitly verify the MCP tool manifests and run the integration tests over MCP. Cleanups are also improved by utilizing t.Cleanup and detaching the cleanup context from the potentially cancelled test context. I have no feedback to provide as there are no review comments.