Breaking change: support last_used/preferred pipeline IDs and use last_used by default

[jpelgrom]

Summary

Update native Assist to better match frontend Assist behavior:

• default to the last used pipeline if not specified instead of the preferred pipeline
• allow the frontend to choose between last_used, preferred, or a specific pipeline -> requires Add last_used option for pipeline and explicit default frontend#17329 / 2023.8

Breaking change, matching frontend: Assist shortcuts will open last used pipeline by default instead of preferred. To use preferred, recreate the shortcut.

Screenshots

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Assist shortcut options updated with 'Last used assistant' (default)

Link to pull request in Documentation repository

n/a

Any other notes