Changes multi-diff editor should preserve collapse state when switching sessions

When juggling multiple agent sessions, the collapsed/expanded state in the **Changes** multi-diff editor is lost after switching away from a session and returning to it.

## Reproduction

1. Open an agent session with multiple files in the Changes multi-diff editor.
2. Collapse some files or sections to track what has already been reviewed.
3. Switch to another session.
4. Return to the original session.

## Expected

The Changes multi-diff editor restores the previous collapse state for that session.

## Actual

The collapse state is reset, making it difficult to keep track of reviewed changes while moving between sessions.