fix(editor): missing signal of optional flat props (#13762)

Close https://github.com/toeverything/AFFiNE/issues/13750

#### PR Dependency Tree


* **PR #13762** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Optional block properties are now supported (e.g., flat-table), with
default values applied automatically when not set.

* **Bug Fixes**
* More reliable initialization and syncing of block properties, ensuring
defaults appear consistently.
* Change notifications now correctly reflect updates to
optional/defaulted properties.

* **Tests**
* Added tests verifying optional property behavior, default application,
syncing, and change events.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->





#### PR Dependency Tree


* **PR #13762** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

blocksuite/framework/store/src/__tests__/block.unit.spec.ts

@@ -51,6 +51,7 @@ const flatTableSchema = defineBlockSchema({
     textCols: {} as Record<string, Text>,
     rows: {} as Record<string, { color: string }>,
     labels: [] as Array<string>,
+    optional: undefined as string | undefined,
   }),
   metadata: {
     role: 'content',
@@ -494,6 +495,16 @@ describe('flat', () => {
     expect(model.props.textCols$.value.a.toDelta()).toEqual([
       { insert: 'test' },
     ]);
+
+    onChange.mockClear();
+    expect(model.props).not.toHaveProperty('optional');
+    expect(model.props).toHaveProperty('optional$');
+    model.props.optional$.value = 'test';
+    expect(model.props.optional).toBe('test');
+    expect(model.props.optional$.value).toBe('test');
+    expect(onChange).toHaveBeenCalledTimes(1);
+    expect(onChange).toHaveBeenCalledWith(expect.anything(), 'optional', true);
+    expect(yBlock.get('prop:optional')).toBe('test');
   });
 
   test('stash and pop', () => {