Add a common way to create HTTP client (#3110)

Ensure User-Agent and originator are always sent.

codex-rs/exec/src/lib.rs

@@ -191,6 +191,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
     let conversation_manager = ConversationManager::new(AuthManager::shared(
         config.codex_home.clone(),
         config.preferred_auth_method,
+        config.responses_originator_header.clone(),
     ));
     let NewConversation {
         conversation_id: _,