fix(core): ai history of new users is always loading, enable online search by default (#11741)

Close [AI-69](https://linear.app/affine-design/issue/AI-69).

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

@@ -235,7 +235,10 @@ export class ChatPanelMessages extends WithDisposable(ShadowlessElement) {
         @scroll=${() => this._debouncedOnScroll()}
       >
         ${filteredItems.length === 0
-          ? html`<div class="messages-placeholder">
+          ? html`<div
+              class="messages-placeholder"
+              data-testid="chat-panel-messages-placeholder"
+            >
               ${AffineIcon(
                 isLoading
                   ? 'var(--affine-icon-secondary)'

packages/frontend/core/src/blocksuite/ai/chat-panel/index.ts

@@ -400,7 +400,7 @@ export class ChatPanel extends SignalWatcher(
         .createSessionId=${this._createSessionId}
         .updateContext=${this.updateContext}
         .host=${this.host}
-        .isLoading=${this.isLoading || !this._isInitialized}
+        .isLoading=${this.isLoading}
         .previewSpecBuilder=${this.previewSpecBuilder}
       ></chat-panel-messages>
       <ai-chat-composer