test(mcp): cover stdio disconnect cleanup

[Harsh Mathur (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

[github-actions]

Test results for "MCP"

1 failed
❌ [firefox] › mcp/cli-parsing.spec.ts:76 › should preserve leading zeros in string arguments @mcp-windows-latest-firefox

8062 passed, 1284 skipped

---

Merge workflow run.