Fixes that file attachments without captions would not be represented in plain text generated from rich-text content, causing ActionText::RichText#present? to return false.
Closes#36607.
Assigning a has_one association for a persisted record saves the change immediately, so attempting to read a rich-text attribute on a persisted record without a corresponding ActionText::RichText would eagerly create one. Avoid assigning the rich text association to fix.
Failing test before the ActionText::RichText change:
```
Error:
ActionText::ModelTest#test_embed_extraction_only_extracts_file_attachments:
ArgumentError: Could not find or build blob: expected attachable, got #<ActionText::Attachables::RemoteImage:0x00007fb0259fef70 @url="http://example.com/cat.jpg", @content_type="image", @width=nil, @height=nil>
```
