fix(ui): use discard for unused cache access in tests

Replace unused `first` variable assignments with discards `_` to
satisfy static analysis.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: JasonXuDeveloper - 傑 <jason@xgamedev.net>

Author: JasonXuDeveloper

UnityProject/Packages/com.jasonxudeveloper.jengine.ui/Tests/Editor/Utilities/StyleSheetManagerTests.cs

@@ -144,7 +144,7 @@ public void ClearCache_CanBeCalledMultipleTimes()
         public void ClearCache_ResetsTokensCache()
         {
             // Access to populate cache
-            var first = StyleSheetManager.Tokens;
+            _ = StyleSheetManager.Tokens;
 
             StyleSheetManager.ClearCache();
 
@@ -159,7 +159,7 @@ public void ClearCache_ResetsTokensCache()
         [Test]
         public void ClearCache_ResetsBaseCache()
         {
-            var first = StyleSheetManager.Base;
+            _ = StyleSheetManager.Base;
             StyleSheetManager.ClearCache();
             var second = StyleSheetManager.Base;
 
@@ -169,7 +169,7 @@ public void ClearCache_ResetsBaseCache()
         [Test]
         public void ClearCache_ResetsComponentsCache()
         {
-            var first = StyleSheetManager.Components;
+            _ = StyleSheetManager.Components;
             StyleSheetManager.ClearCache();
             var second = StyleSheetManager.Components;