Gallery: Hide Navigation button type when lightbox editing is disabled

[ishitaj34]

What?

Closes #79133

This PR hides the "Navigation button type" ToolsPanelItem for the Gallery block when Image block lightbox editing is disabled via theme.json.

Why?

On disabling Image block lightbox functionality using the below code snippet, the Gallery block was still registering the "Navigation button type" panel item in the Settings menu even though there was no way to configure it or interact with it.

{
  "settings": {
    "blocks": {
      "core/image": {
        "lightbox": {
          "allowEditing": false,
          "enabled": false
        }
      }
    }
  }
}

So it was exposing a menu item for a control that was not applicable.

How?

The "Navigation button type" is now only registered when lightboxSetting?.allowEditing is enabled, so it prevents the item from being added to the ToolsPanel menu when lightbox editing is disabled, while preserving the existing behavior when lightbox editing is available.

Testing Instructions

1. Start a Gutenberg development environment.
2. Use a block theme (used Twenty Twenty-Five to reproduce and test).
3. Modify the theme.json to include:

{
  "settings": {
    "blocks": {
      "core/image": {
        "lightbox": {
          "allowEditing": false,
          "enabled": false
        }
      }
    }
  }
}

4. Refresh the editor.
5. Create a post and insert a Gallery block with images.
6. Open the ⋮ menu next to Settings in the block inspector.
7. Verify that Navigation button type is not listed.
8. To check regression, restore the original theme.json or remove the lightbox configuration above.
9. Refresh the editor and open the same Gallery block.
10. Open the ⋮ menu next to Settings.
11. Verify that "Navigation button type" is visible again.

Testing Instructions for Keyboard

1. Open a post and insert a Gallery block.
2. Use the keyboard to select the Gallery block.
3. Navigate to the Settings panel and open the ⋮ menu using the keyboard.
4. With lightbox editing disabled via theme.json, verify that "Navigation button type" is not present in the menu.
5. Restore the default configuration and verify that the option is present again and remains keyboard accessible.

Screenshots or screencast

Before	After

Use of AI Tools

AI assistance was used for code analysis, drafting documentation, and implementation review. The implementation, testing, and verification were performed by author.

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[t-hamano]

Thanks for the PR! This PR works well, but I think we can fix another problem at the same time.

Enable the default theme and insert a gallery block. Even though there are no images with the lightbox enabled, the menu item is displayed.

To fix this, we can simply do the following:

{ lightboxSetting?.allowEditing && hasLightboxImages && (
	<ToolsPanelItem
		label={ __( 'Navigation button type' ) }
	>
	</ToolsPanelItem>
) }