Terminal does not update when switching sessions in Agents Window

### Type of Issue

Bug

### Description

Until recently, switching sessions using the picker on the left side of the Agents Window updated the terminal to show the terminal associated with the newly selected session. This no longer happens: after switching sessions, the terminal continues to show the stale terminal from the previously selected session.

### Steps to Reproduce

1. Open the Agents Window with at least two sessions that have terminals.
2. Select one session using the picker on the left.
3. Switch to another session using the same picker.

### Expected Behavior

The terminal updates to show the terminal associated with the newly selected session.

### Actual Behavior

The terminal continues to show the stale terminal from the previously selected session.

### Regression

This worked until recently.

Start by locating the Agents Window session-picker selection flow and the code that associates its selected session with the displayed terminal. Reproduce the issue with two sessions that have terminals, then trace why switching sessions leaves the previous terminal selected. Done means the terminal changes to the newly selected session and the behavior is covered by the relevant test.

Written by the indexing model from the issue text.