api: fix unreachable status err (#11423)

StatusError was unreachable, the client always checked for error messages in the response body first, and the server always includes error messages with HTTP error status codes.

api/client_test.go

@@ -89,6 +89,16 @@ func TestClientStream(t *testing.T) {
 			},
 			wantErr: "mid-stream error",
 		},
+		{
+			name: "http status error takes precedence over general error",
+			responses: []any{
+				testError{
+					message:    "custom error message",
+					statusCode: http.StatusInternalServerError,
+				},
+			},
+			wantErr: "500",
+		},
 		{
 			name: "successful stream completion",
 			responses: []any{