test(mcp): cover stdio disconnect cleanup

[harshmathurx]

Summary

• add regression coverage for isolated MCP stdio cleanup when the client transport disconnects
• expose the stdio transport from the MCP test fixture so the test can close the transport directly
• make fixture cleanup tolerate an already-closed client after simulated disconnect

Test Plan

• npm run build
• npm run test-mcp -- --project=chromium tests/mcp/launch.spec.ts -g "isolated stdio server closes browser on client transport disconnect"
• npm run test-mcp -- --project=chromium tests/mcp/launch.spec.ts
• npx eslint tests/mcp/fixtures.ts tests/mcp/launch.spec.ts
• npm run tsc

References #41918