[codex] Route Fed ChatGPT auth through Fed edge (#17151)

## Summary
- parse chatgpt_account_is_fedramp from signed ChatGPT auth metadata
- add _account_is_fedramp=true to ChatGPT backend-api requests only for
FedRAMP ChatGPT-auth accounts

codex-rs/codex-api/src/auth.rs

@@ -1,4 +1,5 @@
 use http::HeaderMap;
+use http::HeaderValue;
 
 /// Adds authentication headers to API requests.
 ///
@@ -8,3 +9,26 @@ use http::HeaderMap;
 pub trait AuthProvider: Send + Sync {
     fn add_auth_headers(&self, headers: &mut HeaderMap);
 }
+
+pub(crate) fn add_fedramp_routing_header(headers: &mut HeaderMap) {
+    headers.insert("X-OpenAI-Fedramp", HeaderValue::from_static("true"));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn add_fedramp_routing_header_sets_header() {
+        let mut headers = HeaderMap::new();
+
+        add_fedramp_routing_header(&mut headers);
+
+        assert_eq!(
+            headers
+                .get("X-OpenAI-Fedramp")
+                .and_then(|v| v.to_str().ok()),
+            Some("true")
+        );
+    }
+}