fix: hide internal server error details from users

Signed-off-by: Ivan Zvyagintsev <ivan.zvyagintsev@flant.com>

server/deviceflowhandlers_test.go

@@ -222,8 +222,9 @@ func TestDeviceCallback(t *testing.T) {
 				code:  "somecode",
 				error: "Error Condition",
 			},
-			expectedResponseCode:   http.StatusBadRequest,
-			expectedServerResponse: "Error Condition: \n",
+			expectedResponseCode: http.StatusBadRequest,
+			// Note: Error details should NOT be displayed to user anymore.
+			// Instead, a safe generic message is shown.
 		},
 		{
 			testName: "Expired Auth Code",
@@ -352,8 +353,9 @@ func TestDeviceCallback(t *testing.T) {
 				code:  "somecode",
 				error: "<script>console.log(window);</script>",
 			},
-			expectedResponseCode:   http.StatusBadRequest,
-			expectedServerResponse: "&lt;script&gt;console.log(window);&lt;/script&gt;: \n",
+			expectedResponseCode: http.StatusBadRequest,
+			// Note: XSS data should NOT be displayed to user anymore.
+			// Instead, a safe generic message is shown.
 		},
 	}
 	for _, tc := range tests {
@@ -413,6 +415,29 @@ func TestDeviceCallback(t *testing.T) {
 					t.Errorf("%s: Unexpected Response.  Expected %q got %q", tc.testName, tc.expectedServerResponse, result)
 				}
 			}
+
+			// Special check for error message safety tests
+			if tc.testName == "Prevent cross-site scripting" || tc.testName == "Error During Authorization" {
+				result, _ := io.ReadAll(rr.Body)
+				responseBody := string(result)
+
+				// Error details should NOT be present in the response (for security)
+				if tc.testName == "Prevent cross-site scripting" {
+					if strings.Contains(responseBody, "<script>") || strings.Contains(responseBody, "console.log(window)") {
+						t.Errorf("%s: XSS script found in response, but should be blocked: %q", tc.testName, responseBody)
+					}
+				}
+				if tc.testName == "Error During Authorization" {
+					if strings.Contains(responseBody, "Error Condition") {
+						t.Errorf("%s: Error details found in response, but should be hidden: %q", tc.testName, responseBody)
+					}
+				}
+
+				// Safe message should be present
+				if !strings.Contains(responseBody, "Authorization failed. Please try again.") {
+					t.Errorf("%s: Safe error message not found in response: %q", tc.testName, responseBody)
+				}
+			}
 		})
 	}
 }