Add UiUtilsThemeTest for reflection-based dark theme detection

Cover UiUtils.isDarkTheme() with mocked PlatformUI/Platform statics:
WITH e4 CSS theming (dark id, light id, mixed-case id, and a null active
theme falling back to the preference) and WITHOUT theming (theme bundle
absent, theme service null, and a reflective loadClass failure) — all of
which must degrade to the persisted themeid preference and finally to
light without throwing.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: travkin79

com.microsoft.copilot.eclipse.ui.test/src/com/microsoft/copilot/eclipse/ui/utils/UiUtilsThemeTest.java

@@ -0,0 +1,178 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+package com.microsoft.copilot.eclipse.ui.utils;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.doThrow;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.mockStatic;
+import static org.mockito.Mockito.when;
+
+import org.eclipse.core.runtime.Platform;
+import org.eclipse.core.runtime.preferences.IPreferencesService;
+import org.eclipse.e4.ui.css.swt.theme.ITheme;
+import org.eclipse.e4.ui.css.swt.theme.IThemeEngine;
+import org.eclipse.ui.IWorkbench;
+import org.eclipse.ui.PlatformUI;
+import org.junit.jupiter.api.Test;
+import org.mockito.MockedStatic;
+import org.osgi.framework.Bundle;
+
+/**
+ * Unit tests for {@link UiUtils#isDarkTheme()}.
+ *
+ * <p>The detection resolves the active e4 CSS theme reflectively through the theme bundle's own
+ * class loader, so no compile-time reference to the friend-restricted
+ * {@code org.eclipse.e4.ui.css.swt.theme} package is required in production. These tests drive that
+ * logic entirely through mocked {@link PlatformUI} and {@link Platform} statics, covering both the
+ * case where e4 CSS theming is available and the graceful fallbacks when it is not.
+ */
+class UiUtilsThemeTest {
+
+  private static final String THEME_BUNDLE = "org.eclipse.e4.ui.css.swt.theme";
+  private static final String DARK_THEME_ID = "org.eclipse.e4.ui.css.theme.e4_dark";
+  private static final String LIGHT_THEME_ID = "org.eclipse.e4.ui.css.theme.e4_default";
+
+  // ---------------------------------------------------------------------------
+  // Theming available: active theme resolved via reflection.
+  // ---------------------------------------------------------------------------
+
+  @Test
+  void isDarkTheme_activeThemeIsDark_returnsTrue() throws Exception {
+    try (MockedStatic<PlatformUI> platformUi = mockStatic(PlatformUI.class);
+        MockedStatic<Platform> platform = mockStatic(Platform.class)) {
+      IWorkbench workbench = givenThemingAvailable(platformUi, platform);
+      givenActiveTheme(workbench, DARK_THEME_ID);
+
+      assertTrue(UiUtils.isDarkTheme());
+    }
+  }
+
+  @Test
+  void isDarkTheme_activeThemeIsLight_returnsFalse() throws Exception {
+    try (MockedStatic<PlatformUI> platformUi = mockStatic(PlatformUI.class);
+        MockedStatic<Platform> platform = mockStatic(Platform.class)) {
+      IWorkbench workbench = givenThemingAvailable(platformUi, platform);
+      givenActiveTheme(workbench, LIGHT_THEME_ID);
+
+      assertFalse(UiUtils.isDarkTheme());
+    }
+  }
+
+  @Test
+  void isDarkTheme_activeThemeIdMixedCase_returnsTrue() throws Exception {
+    try (MockedStatic<PlatformUI> platformUi = mockStatic(PlatformUI.class);
+        MockedStatic<Platform> platform = mockStatic(Platform.class)) {
+      IWorkbench workbench = givenThemingAvailable(platformUi, platform);
+      givenActiveTheme(workbench, "com.example.Solarized.DARK.Theme");
+
+      assertTrue(UiUtils.isDarkTheme());
+    }
+  }
+
+  @Test
+  void isDarkTheme_activeThemeNull_fallsBackToPreference() throws Exception {
+    try (MockedStatic<PlatformUI> platformUi = mockStatic(PlatformUI.class);
+        MockedStatic<Platform> platform = mockStatic(Platform.class)) {
+      IWorkbench workbench = givenThemingAvailable(platformUi, platform);
+      givenActiveTheme(workbench, null);
+      givenPersistedThemeId(platform, DARK_THEME_ID);
+
+      assertTrue(UiUtils.isDarkTheme());
+    }
+  }
+
+  // ---------------------------------------------------------------------------
+  // Theming unavailable: graceful fallback to the persisted preference.
+  // ---------------------------------------------------------------------------
+
+  @Test
+  void isDarkTheme_themeBundleAbsent_fallsBackToPreferenceDark() {
+    try (MockedStatic<PlatformUI> platformUi = mockStatic(PlatformUI.class);
+        MockedStatic<Platform> platform = mockStatic(Platform.class)) {
+      platform.when(() -> Platform.getBundle(THEME_BUNDLE)).thenReturn(null);
+      givenPersistedThemeId(platform, DARK_THEME_ID);
+
+      assertTrue(UiUtils.isDarkTheme());
+    }
+  }
+
+  @Test
+  void isDarkTheme_themeBundleAbsent_fallsBackToPreferenceLight() {
+    try (MockedStatic<PlatformUI> platformUi = mockStatic(PlatformUI.class);
+        MockedStatic<Platform> platform = mockStatic(Platform.class)) {
+      platform.when(() -> Platform.getBundle(THEME_BUNDLE)).thenReturn(null);
+      givenPersistedThemeId(platform, "");
+
+      assertFalse(UiUtils.isDarkTheme());
+    }
+  }
+
+  @Test
+  void isDarkTheme_themeServiceNull_fallsBackToPreference() throws Exception {
+    try (MockedStatic<PlatformUI> platformUi = mockStatic(PlatformUI.class);
+        MockedStatic<Platform> platform = mockStatic(Platform.class)) {
+      IWorkbench workbench = givenThemingAvailable(platformUi, platform);
+      doReturn(null).when(workbench).getService(IThemeEngine.class);
+      givenPersistedThemeId(platform, DARK_THEME_ID);
+
+      assertTrue(UiUtils.isDarkTheme());
+    }
+  }
+
+  @Test
+  void isDarkTheme_reflectionFailure_fallsBackToPreferenceWithoutThrowing() throws Exception {
+    try (MockedStatic<PlatformUI> platformUi = mockStatic(PlatformUI.class);
+        MockedStatic<Platform> platform = mockStatic(Platform.class)) {
+      IWorkbench workbench = mock(IWorkbench.class);
+      platformUi.when(PlatformUI::getWorkbench).thenReturn(workbench);
+
+      Bundle themeBundle = mock(Bundle.class);
+      doThrow(new ClassNotFoundException()).when(themeBundle)
+          .loadClass("org.eclipse.e4.ui.css.swt.theme.IThemeEngine");
+      platform.when(() -> Platform.getBundle(THEME_BUNDLE)).thenReturn(themeBundle);
+      givenPersistedThemeId(platform, LIGHT_THEME_ID);
+
+      assertFalse(UiUtils.isDarkTheme());
+    }
+  }
+
+  // ---------------------------------------------------------------------------
+  // Helpers.
+  // ---------------------------------------------------------------------------
+
+  private static IWorkbench givenThemingAvailable(MockedStatic<PlatformUI> platformUi,
+      MockedStatic<Platform> platform) throws ClassNotFoundException {
+    IWorkbench workbench = mock(IWorkbench.class);
+    platformUi.when(PlatformUI::getWorkbench).thenReturn(workbench);
+
+    Bundle themeBundle = mock(Bundle.class);
+    doReturn(IThemeEngine.class).when(themeBundle)
+        .loadClass("org.eclipse.e4.ui.css.swt.theme.IThemeEngine");
+    doReturn(ITheme.class).when(themeBundle)
+        .loadClass("org.eclipse.e4.ui.css.swt.theme.ITheme");
+    platform.when(() -> Platform.getBundle(THEME_BUNDLE)).thenReturn(themeBundle);
+    return workbench;
+  }
+
+  private static void givenActiveTheme(IWorkbench workbench, String themeId) {
+    IThemeEngine themeEngine = mock(IThemeEngine.class);
+    doReturn(themeEngine).when(workbench).getService(IThemeEngine.class);
+
+    ITheme activeTheme = null;
+    if (themeId != null) {
+      activeTheme = mock(ITheme.class);
+      when(activeTheme.getId()).thenReturn(themeId);
+    }
+    when(themeEngine.getActiveTheme()).thenReturn(activeTheme);
+  }
+
+  private static void givenPersistedThemeId(MockedStatic<Platform> platform, String themeId) {
+    IPreferencesService preferences = mock(IPreferencesService.class);
+    platform.when(Platform::getPreferencesService).thenReturn(preferences);
+    when(preferences.getString(THEME_BUNDLE, "themeid", "", null)).thenReturn(themeId);
+  }
+}