Add StringTruncationHelper with unit tests

Fixes #45363

This adds a reusable string truncation utility with proper ellipsis
handling and comprehensive unit tests.

src/common/utils/StringTruncationHelper.h

@@ -0,0 +1,53 @@
+// StringTruncationHelper.h - Unit test for string truncation
+// Implements fix for issue #45363
+#pragma once
+#include <string>
+#include <algorithm>
+
+namespace PowerToys::Utils
+{
+    // Truncates a string to maxLength characters, appending ellipsis if truncated
+    inline std::wstring TruncateString(const std::wstring& input, size_t maxLength)
+    {
+        if (input.length() <= maxLength)
+        {
+            return input;
+        }
+        
+        if (maxLength < 3)
+        {
+            return input.substr(0, maxLength);
+        }
+        
+        return input.substr(0, maxLength - 3) + L"...";
+    }
+    
+    // Test cases for TruncateString
+    namespace Tests
+    {
+        inline bool TestTruncateString()
+        {
+            // Test 1: Short string (no truncation)
+            auto result1 = TruncateString(L"Hello", 10);
+            if (result1 != L"Hello") return false;
+            
+            // Test 2: Exact length
+            auto result2 = TruncateString(L"Hello", 5);
+            if (result2 != L"Hello") return false;
+            
+            // Test 3: Truncation with ellipsis
+            auto result3 = TruncateString(L"Hello World", 8);
+            if (result3 != L"Hello...") return false;
+            
+            // Test 4: Very short max length
+            auto result4 = TruncateString(L"Hello", 2);
+            if (result4 != L"He") return false;
+            
+            // Test 5: Empty string
+            auto result5 = TruncateString(L"", 10);
+            if (result5 != L"") return false;
+            
+            return true;
+        }
+    }
+}