fix(core): old ai messages not cleared before retrying (#13119)

Close [AI-258](https://linear.app/affine-design/issue/AI-258)

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

## Summary by CodeRabbit

* **Bug Fixes**
* Improved the retry behavior in AI chat by ensuring previous streaming
data is properly cleared before retrying, resulting in more accurate and
consistent chat message handling.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/blocksuite/ai/components/ai-chat-messages/ai-chat-messages.ts

@@ -437,6 +437,7 @@ export class AIChatMessages extends WithDisposable(ShadowlessElement) {
       const last = messages[messages.length - 1];
       if ('content' in last) {
         last.content = '';
+        last.streamObjects = [];
         last.createdAt = new Date().toISOString();
       }
       this.updateContext({

packages/frontend/core/src/blocksuite/ai/peek-view/chat-block-peek-view.ts

@@ -372,7 +372,7 @@ export class AIChatBlockPeekView extends LitElement {
       const last = messages[messages.length - 1];
       if ('content' in last) {
         last.content = '';
-        last.id = '';
+        last.streamObjects = [];
         last.createdAt = new Date().toISOString();
       }
       this.updateContext({