fix(chat): keep the diff preview bound off a surrogate pair

The 256 KiB budget for an oversized tool diff preview counts UTF-16 units, so
the cut can land between the two halves of an astral character - an emoji in a
patched string, for instance. The preview then ended in a lone surrogate that
renders as the replacement glyph.

Step back one unit when the boundary splits a pair.

packages/ui/src/components/chat/message/parts/toolDiffPreview.ts

@@ -22,8 +22,20 @@ const getPreviewEnd = (diff: string): number | null => {
 
 export const isToolDiffPreviewOversized = (diff: string): boolean => getPreviewEnd(diff) !== null;
 
+/**
+ * The character budget counts UTF-16 units, so it can land between the two
+ * halves of an astral character (an emoji in a patched string, say). Cutting
+ * there leaves a lone surrogate that renders as the replacement glyph, so step
+ * back one unit when the boundary splits a pair.
+ */
+const withoutSplitSurrogate = (diff: string, previewEnd: number): number => {
+    const lastUnit = diff.charCodeAt(previewEnd - 1);
+    const isHighSurrogate = lastUnit >= 0xd800 && lastUnit <= 0xdbff;
+    return isHighSurrogate ? previewEnd - 1 : previewEnd;
+};
+
 export const getToolDiffPreviewText = (diff: string): string => {
     const previewEnd = getPreviewEnd(diff);
     if (previewEnd === null) return diff;
-    return `${diff.slice(0, previewEnd).trimEnd()}\n…`;
+    return `${diff.slice(0, withoutSplitSurrogate(diff, previewEnd)).trimEnd()}\n…`;
 };

packages/ui/src/components/chat/message/parts/toolDiffUtils.test.ts

@@ -223,6 +223,18 @@ describe('toolDiffUtils', () => {
         expect(preview.length).toBe(TOOL_DIFF_PREVIEW_MAX_CHARS + 2);
     });
 
+    test('does not cut an astral character in half at the character limit', () => {
+        // The boundary lands between the two halves of the emoji, which would
+        // otherwise leave a lone surrogate rendering as the replacement glyph.
+        const patch = `+${'x'.repeat(TOOL_DIFF_PREVIEW_MAX_CHARS - 2)}\u{1F600}${'y'.repeat(100)}`;
+        const preview = getToolDiffPreviewText(patch);
+        const body = preview.slice(0, -2);
+
+        expect(isToolDiffPreviewOversized(patch)).toBe(true);
+        expect(body).toBe(`+${'x'.repeat(TOOL_DIFF_PREVIEW_MAX_CHARS - 2)}`);
+        expect(/[\uD800-\uDFFF]/.test(body)).toBe(false);
+    });
+
     test('preserves diff previews at the character and line limits', () => {
         const characterLimit = 'x'.repeat(TOOL_DIFF_PREVIEW_MAX_CHARS);
         const lineLimit = Array.from({ length: TOOL_DIFF_PREVIEW_MAX_LINES }, () => '+line').join('\n');