multisessions: `+` button does not dedupe prior to limiting to 5

[midleman]

System details:

Positron and OS details:

2025.06.0-145

Describe the issue:

I'm not seeing the last 5 runtimes in the + session context menu.

Steps to reproduce the issue:

1. Start up 5 R sessions (must be the exact same runtime)
2. Start up 1 Python session
3. Click the + button.
4. Verify both runtimes display - PASS
5. Start up 1 Python alternate/different runtime
6. Verify all 3 runtimes display - FAIL (only R and new Python does)
7. Delete the first R runtime
8. Verify all 3 runtimes display - FAIL (now the R one has disappeared) 🤷‍♀

https://github.com/user-attachments/assets/daa34473-ceb7-4f2f-a8e6-71d3959deddd

Expected or desired behavior:

Console + session button should show the last 5 runtimes.