Beta extensions can be added to a project multiple times

Describe the bug
Normally when an extension is added to a user project, it is no longer available in the extension search.
With beta extensions this does not seem to be the case.

CC/Found by @microbit-caroline.

To Reproduce
Steps to reproduce the behavior:

1. Create a new project
2. Go to extensions and search for "microbit-v2-power", multiple extensions should appear, including a beta extension
3. Add it to the project
4. Go to extension and search again for "microbit-v2-power", the extension still appears and it can be added to the project again.

Expected behavior
The extension should not appear in the search after it has been added to the user project.

Screenshots

micro:bit version (please complete the following information):
N/A

Desktop (please complete the following information):

• OS: macOS
• Browser: Chrome
• Version: 138

Smartphone (please complete the following information):
N/A

Additional context
N/A

Reproduce the issue by creating a project, adding the beta “microbit-v2-power” extension, and searching for it again in the Extensions view. Trace the search results and project-extension state, then verify that an already-added beta extension no longer appears or can be added again.