The "Show more files" button replaces `#diff-incomplete` with newly
loaded diff file boxes.
The inserted HTML may contain htmx attributes, but they are not
processed after insertion.
### Solution
Wrap the incomplete diff placeholder with a temporary wrapper so we can
call `htmx.process()` on the newly inserted content.
After processing, unwrap the wrapper to keep the DOM structure
unchanged.
### Testing
- Open a large PR diff page where `Diff.IsIncomplete` is true
- Click "Show more files"
- Verify newly loaded file boxes behave correctly (htmx-related features
work as expected)
<img width="927" height="278" alt="image"
src="https://github.com/user-attachments/assets/54f2b4f2-c0e1-483c-9e26-79a2838e98ee"
/>
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
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