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

internal/chatcmpl/client.go

@@ -42,6 +42,33 @@ func (c *Client) endpoint() string {
 	return c.BaseURL + "/chat/completions"
 }
 
+// ListModels fetches the upstream model catalog. Not every provider
+// implements it; callers should treat an error as "list unavailable"
+// and fall back to manual entry.
+func (c *Client) ListModels(ctx context.Context) (*ModelList, error) {
+	httpReq, err := http.NewRequestWithContext(ctx, http.MethodGet, c.BaseURL+"/models", nil)
+	if err != nil {
+		return nil, err
+	}
+	httpReq.Header.Set("Accept", "application/json")
+	if c.APIKey != "" {
+		httpReq.Header.Set("Authorization", "Bearer "+c.APIKey)
+	}
+	resp, err := c.HTTP.Do(httpReq)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return nil, &UpstreamError{StatusCode: resp.StatusCode, Message: readErrorBody(resp)}
+	}
+	var out ModelList
+	if err := json.NewDecoder(io.LimitReader(resp.Body, 8<<20)).Decode(&out); err != nil {
+		return nil, fmt.Errorf("decode upstream model list: %w", err)
+	}
+	return &out, nil
+}
+
 func (c *Client) do(ctx context.Context, req *Request) (*http.Response, error) {
 	body, err := json.Marshal(req)
 	if err != nil {