test(source/mindsdb): migrate integration tests to MCP endpoint - #4200

Description

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

Key Changes

• mindsdb_mcp_test.go: adds TestMindsDBMCPListTools (validates the tools/list
  manifest) and TestMindsDBMCPToolEndpoints (runs RunToolInvokeTest withWithMCP() and the same MindsDB SQL, integration, error-handling and auth checks as the legacy test over MCP).
• mindsdb_integration_test.go: replaces the REST-only RunToolInvokeSimpleTest/RunToolInvokeParametersTest calls with a local REST/MCP transport so both tests run the same checks; the shared helpers in tests/tool.go are unchanged, and the legacy assertions are unchanged.
• Cleanup hardening: the connection pool is closed, table drops are registered before the tables are created (with a bounded timeout), and REST requests use the test context.

Testing

go vet passes. The integration tests need a live MindsDB instance and a Google ID token, so they will first run in CI via the mindsdb shard.

PR Checklist

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🛠️ Fixes #<issue_number_goes_here>