file_write: ensure newline before closing diff fence

tools/file_write

@@ -23,6 +23,8 @@ def format_diff(path, old, new):
     diff = ''.join(difflib.unified_diff(old_lines, new_lines, fromfile=path, tofile=path))
     if not diff:
         return None
+    if not diff.endswith('\n'):
+        diff += '\n'
     return f"```diff\n{diff}```"
 
 if len(sys.argv) < 3: