fix(core): cannot input space at the beginning of a blank paragraph (#12166)

### TL:DR

fix: cannot input space at the beginning of a blank paragraph

> CLOSE BS-3427

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

- **New Features**
  - Improved space key handling in the editor: pressing space on an empty AI input now hides the AI panel and inserts a space character back into the editor.

- **Bug Fixes**
  - Prevented the AI panel from processing empty input when space is pressed, ensuring smoother user experience.

- **Tests**
  - Added an end-to-end test verifying that pressing space on an empty AI input hides the AI panel and inserts a space.

- **Refactor**
  - Streamlined event handling logic for space key detection in the editor.

- **Chores**
  - Enhanced editor content retrieval to optionally preserve whitespace while removing invisible characters.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/blocksuite/ai/entries/space/setup-space.ts

@@ -1,9 +1,34 @@
-import { TextSelection } from '@blocksuite/affine/std';
+import type { RichText } from '@blocksuite/affine/rich-text';
+import { type EditorHost, TextSelection } from '@blocksuite/affine/std';
 
 import { handleInlineAskAIAction } from '../../actions/doc-handler';
 import { AIProvider } from '../../provider';
 import type { AffineAIPanelWidget } from '../../widgets/ai-panel/ai-panel';
 
+function isSpaceEvent(event: KeyboardEvent) {
+  return event.key === ' ' && event.which === 32 && !event.isComposing;
+}
+
+function insertSpace(host: EditorHost) {
+  const textSelection = host.selection.find(TextSelection);
+  if (!textSelection || !textSelection.isCollapsed()) return;
+
+  const blockComponent = host.view.getBlock(textSelection.from.blockId);
+  if (!blockComponent) return;
+
+  const richText = blockComponent.querySelector('rich-text') as RichText | null;
+  if (!richText) return;
+
+  const inlineEditor = richText.inlineEditor;
+  inlineEditor?.insertText(
+    {
+      index: textSelection.from.index,
+      length: 0,
+    },
+    ' '
+  );
+}
+
 export function setupSpaceAIEntry(panel: AffineAIPanelWidget) {
   // Background: The keydown event triggered by a space may originate from:
   // 1. Normal space insertion
@@ -18,12 +43,19 @@ export function setupSpaceAIEntry(panel: AffineAIPanelWidget) {
     const host = panel.host;
     const keyboardState = ctx.get('keyboardState');
     const event = keyboardState.raw;
-    if (
-      AIProvider.actions.chat &&
-      event.key === ' ' &&
-      event.which === 32 &&
-      !event.isComposing
-    ) {
+    if (AIProvider.actions.chat && isSpaceEvent(event)) {
+      // If the AI panel is in the input state and the input content is empty,
+      // insert a space back into the editor.
+      if (panel.state === 'input') {
+        const input = panel.shadowRoot?.querySelector('ai-panel-input');
+        if (input?.textarea.value.trim() === '') {
+          event.preventDefault();
+          insertSpace(host);
+          panel.hide();
+          return;
+        }
+      }
+
       const selection = host.selection.find(TextSelection);
       if (selection && selection.isCollapsed() && selection.from.index === 0) {
         const block = host.view.getBlock(selection.blockId);