Use per-action min-widths for short agent-host pickers (#318502)

Convert the agent-host short-picker set to a map so each picker can have
its own min-width. The tunnel-sharing toggle has no chevron, so it
collapses to 16px instead of 22px.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>