fix(client): stop counting empty edit sides as changed lines

A pure deletion (`new_text: ""`) rendered a blank green row and reported one
addition, because `''.split('\n')` yields one element. A trailing newline did
the same at the end of either side. Both sides now go through one helper that
treats an empty side as no rows and a trailing newline as a terminator. The
defect predates the structural builder; it was carried over from the string
preview it replaced.

client/src/components/Chat/Messages/Content/Parts/DiffView.tsx

@@ -92,6 +92,15 @@ export interface TextEditPreview {
   newText: string;
 }
 
+/**
+ * Rows one side of an edit contributes. An empty side means "nothing here",
+ * not one blank line, and a trailing newline terminates the last line rather
+ * than starting an empty one: plain `split('\n')` would render a blank row and
+ * count it as a change, so a pure deletion (`new_text: ""`) claimed one
+ * addition.
+ */
+const previewRows = (text: string): string[] => (text ? text.replace(/\n$/, '').split('\n') : []);
+
 /**
  * Builds the diff for an `edit_file` args preview straight from the structured
  * edits, instead of formatting them into diff text for `parseUnifiedDiff` to
@@ -116,11 +125,11 @@ export function buildEditPreviewDiff(edits: TextEditPreview[]): ParsedDiff {
     if (index > 0) {
       lines.push({ type: 'hunk', text: '' });
     }
-    for (const text of edit.oldText.split('\n')) {
+    for (const text of previewRows(edit.oldText)) {
       deletions += 1;
       lines.push({ type: 'del', text });
     }
-    for (const text of edit.newText.split('\n')) {
+    for (const text of previewRows(edit.newText)) {
       additions += 1;
       lines.push({ type: 'add', text });
     }

client/src/components/Chat/Messages/Content/Parts/__tests__/DiffView.test.ts

@@ -69,6 +69,24 @@ describe('parseUnifiedDiff', () => {
     expect(parsed.lines.map((line) => line.text)).toEqual(['a', 'b', 'c']);
   });
 
+  it('does not count an empty replacement as an added line', () => {
+    /** A pure deletion (`new_text: ""`) must not render a blank green row or
+     *  report an addition; `''.split('\n')` yields one element. */
+    const parsed = buildEditPreviewDiff([{ oldText: 'gone', newText: '' }]);
+
+    expect(parsed.deletions).toBe(1);
+    expect(parsed.additions).toBe(0);
+    expect(parsed.lines).toEqual([{ type: 'del', text: 'gone' }]);
+  });
+
+  it('treats a trailing newline as a terminator, not an empty final line', () => {
+    const parsed = buildEditPreviewDiff([{ oldText: 'a\n', newText: 'b\n' }]);
+
+    expect(parsed.deletions).toBe(1);
+    expect(parsed.additions).toBe(1);
+    expect(parsed.lines.map((line) => line.text)).toEqual(['a', 'b']);
+  });
+
   it('keeps changed lines that look like the old synthetic header names', () => {
     /** Deleting a real `-- old_text` comment (SQL, Lua) and adding
      *  `++ new_text` prefixes to exactly the text the removed separators used.