fix(ws): skip writing to closed socket on non-upgrade response

[pi0]

Summary

• Check socket.writable before writing the HTTP response when the upstream does not upgrade the WebSocket connection
• Prevents uncatchable "This socket has been ended by the other party" errors when the client disconnects before the non-upgrade response arrives
• Applied to both ws-incoming.ts (ProxyServer middleware) and ws.ts (standalone proxyUpgrade)

Upstream: http-party/node-http-proxy#1552

Test plan

• Regression test added in test/ws.test.ts — verifies socket.write() is not called when the socket is destroyed before the non-upgrade response
• Test fails without the fix, passes with it
• Full test suite passes (229 tests)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.55%. Comparing base (8724245) to head (7a7e97b).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
+ Coverage   96.14%   97.55%   +1.40%     
==========================================
  Files           8        8              
  Lines         623      695      +72     
  Branches      233      279      +46     
==========================================
+ Hits          599      678      +79     
+ Misses         24       17       -7     

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