This PR adds marketplace upgrade to the `/plugins` menu so users can
update configured marketplaces. It adds a `Ctrl+U` shortcut on eligible
marketplace tabs, a loading state, and the app-server request flow
needed to perform `marketplace/upgrade`. After a successful upgrade, the
TUI refreshes plugin data, plugin mentions, and user config so updated
marketplace contents show up across the menu and other plugin surfaces.
It also preserves the current marketplace tab on no-op and failure paths
and surfaces backend error details directly in the TUI.
- Add a `Ctrl+U` upgrade option for user-configured marketplace tabs in
`/plugins`
- Show the upgrade footer hint only on upgradeable marketplace tabs
- Show a loading state during `marketplace/upgrade`
- Surface already-up-to-date and per-marketplace failure results from
the backend
- Refresh plugin data, plugin mentions, and user config after successful
upgrades
- Add tests and snapshot updates for the shortcut flow, loading state,
and failure messaging
Steps to test:
1. Add a `/plugin` marketplace to Codex TUI.
2. Open `/plugins`, move to that marketplace tab, and confirm the footer
shows `Ctrl+U` to upgrade.
3. Press `Ctrl+U` and confirm the popup switches into an upgrade loading
state.
4. When the request finishes, confirm you see the expected result:
updated marketplace contents on success, an already-up-to-date message
on no-op, or backend error details on failure. On no-op or failure,
confirm the popup stays on the same marketplace tab.
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