enhance(http): Support websocket upgrades

[LaurenceJJones]

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Description

Preserve optional ResponseWriter interfaces through statusCapture so httputil.ReverseProxy can hijack upgraded websocket connections for private http proxy. Add a regression test covering websocket traffic through the HTTP handler path.

How to test?

Added a golang test for regression but any private http application that uses websockets should now upgrade via the proxy.

Without exposing the funcs

INFO: 2026/04/29 07:20:27 HTTP handler: GET /socket?token=test -> http://127.0.0.1:40649
ERROR: 2026/04/29 07:20:27 HTTP handler: upstream error (GET /socket?token=test -> http://127.0.0.1:40649): can't switch protocols using non-Hijacker ResponseWriter type *netstack2.statusCapture
 --- FAIL: TestHTTPHandlerProxiesWebSocketUpgrade (0.00s)
http_handler_test.go:79: dial websocket through proxy: websocket: bad handshake
FAIL
FAIL  github.com/fosrl/newt/netstack2 0.004s
FAIL

with now succeeds

ok    github.com/fosrl/newt/netstack2 0.004s

[oschwartz10612]

LGTM and works!