fix(core): chat block opens and throws an error (#12407)

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

## Summary by CodeRabbit

- **Bug Fixes**
  - Improved stability of the AI chat input to prevent errors when certain configuration options are missing.

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

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

@@ -274,7 +274,7 @@ export class AIChatInput extends SignalWatcher(WithDisposable(LitElement)) {
   accessor reasoningConfig!: AIReasoningConfig;
 
   @property({ attribute: false })
-  accessor modelSwitchConfig!: AIModelSwitchConfig;
+  accessor modelSwitchConfig: AIModelSwitchConfig | undefined = undefined;
 
   @property({ attribute: false })
   accessor docDisplayConfig!: DocDisplayConfig;
@@ -394,7 +394,7 @@ export class AIChatInput extends SignalWatcher(WithDisposable(LitElement)) {
           ${ImageIcon()}
           <affine-tooltip>Upload</affine-tooltip>
         </div>
-        ${this.modelSwitchConfig.visible.value
+        ${this.modelSwitchConfig?.visible.value
           ? html`
               <ai-chat-models
                 class="chat-input-icon"