fix(server): keep stdio SSE proxy alive after disconnect

[AkhilTrivediX]

Summary

• handle rejected SSE notification sends from the stdio stderr bridge
• add a server regression test that reproduces an SSE client disconnect while the upstream stdio process continues writing to stderr
• run the server regression suite as part of the standard test command

Root cause

When an Inspector SSE client disconnected, a later stderr notification from the proxied stdio server could call SSEServerTransport.send() after the transport had closed. Its rejected promise was unhandled, so Node exited with Error: Not connected.

The proxy now records the failed notification delivery while keeping the transport lifecycle running. The disconnected client still receives no message, but the Inspector process remains available for subsequent connections.

Closes #1014.

Testing

• npm run test-server
• cd client && npm test -- --runInBand
• npm run build
• npx prettier --check package.json server/package.json server/src/index.ts server/test/stdio-sse-close.test.ts