Add upstream model list fetching and a Codex-specific model section

internal/chatcmpl/types.go

@@ -102,6 +102,18 @@ type Error struct {
 	Code    any    `json:"code,omitempty"`
 }
 
+// ModelInfo is one entry of an upstream model catalog.
+type ModelInfo struct {
+	ID      string `json:"id"`
+	Object  string `json:"object,omitempty"`
+	OwnedBy string `json:"owned_by,omitempty"`
+}
+
+// ModelList is the upstream /models response body.
+type ModelList struct {
+	Data []ModelInfo `json:"data"`
+}
+
 // StreamChunk is one SSE data payload of a streaming response.
 type StreamChunk struct {
 	ID      string        `json:"id"`