fix(ai-vterm): force buffer swap after F9 toggle-off in lone window

The original fix (c558951) set a flag at toggle-off and let the next toggle-on detect it.  The flag mechanism is right, but the toggle-off itself wasn't observable when bury-buffer couldn't switch the lone window onto a different buffer -- `bury-buffer' falls back to `switch-to-prev-buffer', which no-ops when the window's prev-buffer history contains only the agent itself (common right after a `C-x 1' that cleared the surrounding windows' histories).  Without an observable swap, the second F9 found the agent still displayed and routed back through toggle-off, looping the user with no visible effect.

Dispatcher now explicitly forces the window onto another buffer (`(other-buffer agent-buf t)`) when the lone window is still showing the agent after `bury-buffer'.  The round-trip test now exercises the real `bury-buffer' path instead of simulating it; a new test asserts the lone window's buffer is non-agent after toggle-off.

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