fix: restrict CORS headers to localhost origins instead of wildcard

[devin-ai-integration]

Summary

The helper server's HTTP responses included Access-Control-Allow-Origin: * in all 4 CORS emission points — the inline OPTIONS/general responses in handle_helper_connection, plus write_http_response and write_http_stream_headers. While the server already binds to 127.0.0.1 (loopback only), the wildcard CORS policy allows any web page (e.g. a malicious site a user visits) to make cross-origin requests to the local helper, potentially exfiltrating data or triggering actions.

Fix: Three new functions replace the hardcoded *:

fn cors_allowed_origin(request: &str) -> String
// Extracts Origin header, returns it only if is_local_origin() passes

fn is_local_origin(origin: &str) -> bool
// Whitelists: 127.0.0.1, localhost, [::1], ::1, and \"null\" (file:// origins)

fn cors_headers_for_origin(allowed_origin: &str) -> String
// Emits Access-Control-Allow-Origin + Vary: Origin, or empty string for rejected origins

The allowed_origin is computed once in handle_helper_connection and threaded through handle_models_proxy_connection, handle_protocol_proxy_connection, handle_chat_completions_proxy_connection, write_http_response, and write_http_stream_headers. Non-local origins receive no CORS headers (browser blocks the request).

Link to Devin session: https://app.devin.ai/sessions/dff844c4d3f9469e8748f06ca1cecd92
Requested by: @ManhND226677

[devin-ai-integration]

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