`_handle_delete_last` built its deletion as `"\b" * len(text)` and
passed it to `inject_text`, treating U+0008 as typed input. No backend
delivers that as a keypress:
- `ydotool type` has no keymap entry for U+0008, so it types nothing.
- IBus `commit_text()` commits it as a literal control character, which
the client inserts or drops rather than treating as a deletion.
Only an actual key event deletes, so the "delete that" voice command
did nothing on those paths while still reporting success.
Add `press_backspace(count)`, which sends `count` real BackSpace events
using the tool that fits the environment: `xdotool key --repeat` on X11
and the xdotool-backed Wayland path, `wtype -k BackSpace` or the
ydotool keycode pair `14:1 14:0` otherwise. WAYLAND_IBUS resolves a
virtual-keyboard tool of its own, since IBus cannot send key events.
`inject_text` is untouched; only the delete_last action changes.
The existing delete_last tests asserted the U+0008 string was injected,
so they now pin the new contract instead: press_backspace called once
with the character count, and inject_text not called at all.
