Match Bun WebSocket upgrade request headers

Parent: #66

Port Bun corpus position 4220 (js/web/websocket/websocket-upgrade.test.ts).

Scope:

• expose connection: upgrade on the server-side Request
• retain upgrade: websocket
• retain sec-websocket-version: 13
• provide a string sec-websocket-key
• preserve the listener Host header
• keep the exact upstream file in the permanent focused WebSocket gate

Acceptance: 1 passed, 0 todo, 0 failed, 0 unsupported, 0 allowed-empty.

Implementation: 2717608

Implemented on main in 2717608.

Verified the exact Bun corpus file: 1 passed, 0 todo, 0 failed, 0 unsupported, 0 allowed-empty. The complete focused WebSocket/native regression selection also passes.