test(source/dataproc): migrate integration tests to MCP endpoint - #4199

Description

This PR migrates the Dataproc integration tests to use the /mcp endpoint on the MCP
Toolbox Server.

Key Changes

• dataproc_mcp_test.go: adds TestDataprocMCPListTools (validates the tools/list manifest) and TestDataprocClustersMCPToolEndpoints (runs the same cluster and job checks as the legacy test over MCP, including auth: 200 with a valid token, 401 with an invalid or missing token; error cases are asserted as MCP error results).
• dataproc_integration_test.go: replaces invokeTool with a small REST/MCP transport so both tests run the same checks; the legacy assertions are unchanged.
• Cleanup hardening: Dataproc clients and the server are cleaned up with t.Cleanup, and REST requests use the test context.

Testing

go vet passes. The integration tests need Google Cloud credentials and existing clusters and jobs, so they will first run in CI via the dataproc shard.

PR Checklist

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🛠️ Fixes #<issue_number_goes_here>