Fix: Fixed issue where thumbnails didn't refresh when file was updated

[workbysaran]

Resolved / Related Issues
Fixed an issue where files saved to an open folder by external applications (e.g. exporting a PDF from Microsoft Word) would show only the generic file type icon instead of the actual thumbnail.

Closes #18352

Steps used to test these changes

1. Open any folder in Grid View
2. Open an external application (e.g., Microsoft Word), create a document with a few pages, and export or save it as a PDF in the open folder
3. Verify the PDF thumbnail appears correctly without needing to refresh
4. Repeat with other file types and applications
5. Verify files copied via File Explorer still show thumbnails correctly

[yair100]

What's the underlying issue?

[workbysaran]

When an external app (especially one that takes longer to finish writing) writes a file, the Windows thumbnail cache isn’t ready when the file first appears. The initial fetch returns null because the file is still being written, and later modify events don’t refresh the thumbnail.

Fix: If the first fetch returned null, the item was marked NeedsDelayedThumbnailLoad and retried on FILE_ACTION_MODIFIED events with a 500 ms debounce timer.

[yair100]

I need to do a more thorough review, but it looks good at first glance.

[yair100]

Thank you!

[yamachari2025-ops]

The bug where PDF previews do not update has not been fixed.
4.0.51.0
10.0.22631.6199