Close MCP stdio transport on stdin close

[liyanqing90]

Summary

• close the MCP stdio transport when stdin emits either end or close
• make the close path idempotent so both events can fire safely
• add an MCP launch regression test that verifies browser cleanup after stdio client disconnect

Why

The stdio transport only mapped stdin end to transport.close(). Some client disconnect paths surface as stdin close; in that case backend disposal could rely on the broader exit watchdog instead of the normal server close path. Closing the transport on both events lets the MCP server dispose its backend and close the launched browser promptly.

Tests

• PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD=1 npm ci
• npm run build
• npm run tsc
• npm run test-mcp -- launch.spec.ts --project=chrome

[liyanqing90]

@microsoft-github-policy-service agree

[pavelfeldman]

Please start with filing an issue

[liyanqing90]

Filed issue as requested: #41664