Return focus to the agents overview composer on Escape - #44360

## Why

Pressing `Esc` in the agents overview task list dismissed the overview or
exited Codex when no primary thread existed, instead of returning to the draft.

## What changed

Make list cancellation return focus to the composer, including while
reconnecting, preserving the draft and cursor position.

## Testing

Add regression coverage for returning from the list to the composer with Vim
mode enabled or disabled, both online and reconnecting. Verify draft and cursor
preservation, resumed text entry, and the absence of app events.

GitOrigin-RevId: 8920677c42b0c9ff5023979e198f069abcfcf54b