Set user-agent (#2230)

Use the same well-defined value in all cases when sending user-agent
header

codex-rs/core/src/user_agent.rs

@@ -0,0 +1,36 @@
+const DEFAULT_ORIGINATOR: &str = "codex_cli_rs";
+
+pub fn get_codex_user_agent(originator: Option<&str>) -> String {
+    let build_version = env!("CARGO_PKG_VERSION");
+    let os_info = os_info::get();
+    format!(
+        "{}/{build_version} ({} {}; {})",
+        originator.unwrap_or(DEFAULT_ORIGINATOR),
+        os_info.os_type(),
+        os_info.version(),
+        os_info.architecture().unwrap_or("unknown"),
+    )
+}
+
+#[cfg(test)]
+#[allow(clippy::unwrap_used)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_get_codex_user_agent() {
+        let user_agent = get_codex_user_agent(None);
+        assert!(user_agent.starts_with("codex_cli_rs/"));
+    }
+
+    #[test]
+    #[cfg(target_os = "macos")]
+    fn test_macos() {
+        use regex_lite::Regex;
+        let user_agent = get_codex_user_agent(None);
+        let re =
+            Regex::new(r"^codex_cli_rs/\d+\.\d+\.\d+ \(Mac OS \d+\.\d+\.\d+; (x86_64|arm64)\)$")
+                .unwrap();
+        assert!(re.is_match(&user_agent));
+    }
+}