fix(builder): update_props no longer wipes style on style: null

style: null fell through the shallow-merge branch's `value && typeof
value === 'object'` guard and hit the `p[key] = value` fallback, setting
p.style = null and losing every existing style key. Treat style: null (or
any other non-object style value) as "ignore this key" instead.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/utils/apply-ai-response.test.ts

@@ -188,6 +188,36 @@ describe('applyPatch', () => {
     expect(propsById['craft-1'].style).toEqual({ color: 'blue', fontSize: '16px' });
   });
 
+  test('update_props: style: null does NOT wipe existing style (existing keys preserved)', () => {
+    const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    const { query, actions, propsById } = makeSingleNodeHarness('craft-1', 'ai-node-1', {
+      text: 'Old',
+      style: { color: 'red', fontSize: '16px' },
+    });
+    const ops = [
+      {
+        op: 'update_props',
+        node_id: 'ai-node-1',
+        props: { style: null },
+      },
+    ];
+    const result = __test.applyPatch(actions, query, ops);
+    expect(result.ok).toBe(true);
+    expect(propsById['craft-1'].style).toEqual({ color: 'red', fontSize: '16px' });
+    warnSpy.mockRestore();
+  });
+
+  test('update_props: a valid style object still merges shallowly alongside a style:null-safe path', () => {
+    const { query, actions, propsById } = makeSingleNodeHarness('craft-1', 'ai-node-1', {
+      style: { color: 'red', fontSize: '16px' },
+    });
+    const ops = [
+      { op: 'update_props', node_id: 'ai-node-1', props: { style: { color: 'blue' } } },
+    ];
+    __test.applyPatch(actions, query, ops);
+    expect(propsById['craft-1'].style).toEqual({ color: 'blue', fontSize: '16px' });
+  });
+
   test('an op with an unknown resolvedName tree is skipped without throwing; other ops in the batch still apply', () => {
     const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
     const { query, actions, propsById } = makeSingleNodeHarness('craft-1', 'ai-node-1', {});