What?

Fixes #76101

Allows blocks like Post Title/Content to be selectable when inside Template Parts and when 'Show template' is active.

Why?

From my investigation there were two causes of the bug:

1. Template Parts: allow selection in page content focus mode #60010 disables all blocks inside template parts, even Post Title, Post Content and Featured Image. That means those block can't be interacted with.
2. TemplateContentPanel: Don't show content blocks that are in a Query Loop #63732 introduced a getPostBlocksByName selector which is used to find blocks like Post Tile, Post Content, Featured Image and make them editable when 'Show template' is active. The memoization of this selector means it doesn't recompute when controlled blocks like template parts update (because getBlocks itself doesn't, the function has a note that mentions this.), so this selector often doesn't return the post content blocks that are within template parts.

After solving those a third issue was present:

3. Even after the above two fixes, there's another problem. The pattern editing changes apply contentOnly mode to template parts and restricts what can be inserted into post content. This doesn't really make sense since the 'Show template' option is still supposed to be a full post/page editor, but with the template shown. A user should have full post/page editing functionality.

How?

1. Avoids setting blockEditingMode: disable on blocks like Post Content, ensure that the default block editing mode is set instead.
2. Remove the memoization of getPostBlocksByName to ensure blocks can be selected.
3. When 'Show template' is active, template parts are no longer considered contentOnly sections. This isn't a simple change, the 'Show template' mode is implemented in the Editor package while contentOnly sections are in the Block Editor package. To solve this we introduce a disableContentOnlyForTemplateParts editor setting that the editor package sets to true whenever renderingMode === 'template-locked'. This setting is similar to the one introduced for unsynced patterns in Block Editor: Allow disabling content-only editing for unsynced patterns #75457

Testing Instructions

1. Create a template part that has Post Content nested within it
2. Add this template part into a Single Post template
3. Edit a post that uses the template
4. Enable 'Show template' from the document settings
5. Try editing the post content