`Open Preview` does not work for prompt files or custom instructions

[pierceboggan]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: VS Code 1.102
• OS Version: macOS 15.5

Steps to Reproduce:

1. Open VS Code Insiders.
2. Right-click a Markdown file. "Open Preview" should show as an option in the context menu.
3. Right-click a Prompt File or Instruction.

Expected Behavior:

Open Preview shows

Actual Behavior:

Open Preview option is not shown

Comments:

• Developer wants to preview these files as they can be quite unwiedly to view in the regular code view with links, image references, etc.

[mjbvz]

Duplicate of #251049