Handle reserved WebSocket close codes in proxy

[iori0311]

This fixes a WebSocket proxy crash caused by forwarding reserved close codes.

In production I saw close code 1006 coming from one side of the proxy. The current code forwards that code directly with ws.close(event.code, ...), which throws because 1006 is reserved and must not be sent in a close frame.

What changed:

• add close-code validation for outbound close frames
• use a safe fallback code when the incoming code is not sendable
• reuse the same safe close path in close/error handlers

Result:

• no more TypeError: Invalid WebSocket close code: 1006
• proxy no longer hangs after this failure mode

Validation:

• npm run typecheck

Fixes #260