fix(android-webview): prevent content loss on Enter after heading formatting

[egoing]

• Add isMenuVisible parameter to SuggestionMenu keyboard handlers
• Prevent keyboard event processing when menu is not visible
• Strengthen validation for menu state and DOM existence during Enter key handling
• Apply consistent logic to both GridSuggestionMenu and regular SuggestionMenu
• Add selected index validity checks

This change prevents the unintended activation of suggestion menu keyboard handlers when pressing Enter after applying title or other formatting in Android WebView environments, which was causing content to disappear. By ensuring keyboard events are only processed when the menu is actually displayed and exists in the DOM, this improves the stability of user input handling.

Testing approach:

1. Open BlockNote editor in Android WebView
2. Input text and apply title formatting (H1, H2, etc.)
3. Press Enter while formatting is applied
4. Verify that existing content doesn't disappear and a new line is created
5. Confirm that Enter key selection works properly when suggestion menu is displayed
6. Perform regression testing across various browser environments

[vercel]

@egoing is attempting to deploy a commit to the TypeCell Team on Vercel.

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[egoing]

In the WebView, after selecting a format and pressing Enter, the content disappears like this.

3.mov