fix chat highlights (#92589)

## About The Pull Request
If we always define the fragment, the if after the loop will always
pass.
## Why It's Good For The Game
fixes #92468
<img width="653" height="223" alt="grafik"
src="https://github.com/user-attachments/assets/7995b73d-f648-4cbd-b05e-c36f98cab47b"
/>
## Changelog
🆑
fix: chat highlights
/🆑

tgui/packages/tgui-panel/chat/replaceInTextNode.ts

@@ -27,9 +27,9 @@ function regexParseNode(params: ReplaceInTextNodeParams): {
     return { nodes: [], n: 0 };
   }
 
-  const fragment = document.createDocumentFragment();
   const nodes: Node[] = [];
   const textLength = text.length;
+  let fragment: Node | undefined;
   let count = 0;
   let lastIndex = 0;
   let match: RegExpExecArray | null;
@@ -44,6 +44,10 @@ function regexParseNode(params: ReplaceInTextNodeParams): {
     if (++count > 9999) {
       return { nodes: [], n: 0 };
     }
+    // Lazy init fragment
+    if (!fragment) {
+      fragment = document.createDocumentFragment();
+    }
 
     const matchText = captureAdjust ? captureAdjust(match[0]) : match[0];
     const matchLength = matchText.length;