feat: support proxy for ws connection (#9409)

unfortunately tokio-tungstenite doesn't support proxy configuration
outbox, while https://github.com/snapview/tokio-tungstenite/pull/370 is
in review, we can depend on source code for now.

codex-rs/core/src/default_client.rs

@@ -169,7 +169,8 @@ pub fn build_reqwest_client() -> reqwest::Client {
     let mut builder = reqwest::Client::builder()
         // Set UA via dedicated helper to avoid header validation pitfalls
         .user_agent(ua)
-        .default_headers(headers);
+        .default_headers(headers)
+        .use_rustls_tls();
     if is_sandboxed() {
         builder = builder.no_proxy();
     }