Agents: Show All Terminals does not reveal hosted terminals when the panel is hidden

While investigating terminal cleanup in the Agents Window, `Dump Terminal Tracking` showed several terminals as `foreground` even though no terminal UI was visible. Running `Show All Terminals` did not reveal anything.

Steps to reproduce:

1. Open the Agents Window and activate a few sessions so terminals are created.
2. Navigate back to the new-session view and hide the Terminal panel.
3. Run `Dump Terminal Tracking`.
4. See terminals reported as `foreground`.
5. Run `Show All Terminals`.

The command does not reveal the terminals or open the Terminal panel.

`foreground` currently means the terminal is attached to a terminal group/editor rather than stored in the background-terminal collection. It does not mean that the terminal is actually visible. `Show All Terminals` only processes terminals outside `foregroundInstances`, so it is a no-op in this state.

I would expect `Show All Terminals` to make every terminal visible, or for the command/diagnostic terminology to make the narrower behavior clear.