fix(clipboard): keep paste-injection copies out of clipboard history 🤖🤖🤖 (#956)

* fix(clipboard): keep paste-injection copies out of clipboard history

The clipboard-paste injection path writes the dictated text to the
clipboard only to feed the paste chord, then restores the previous
content (#588, #646). Clipboard managers such as Klipper and cliphist
still record that transient copy, so every dictation lands in history.

Mark the injection copy with `wl-copy --sensitive`, which clipboard
managers honour by skipping the entry. The flag is probed once via
`wl-copy --help` (added in wl-clipboard 2.2) so older builds keep
working. It is only set when Copy to Clipboard is off: with that
setting on, the dictated text is meant to stay, so it remains an
ordinary copy. Restoring the user's previous content is never marked
sensitive. xclip/xsel have no equivalent and are unchanged.

* test(clipboard): address review on sensitive paste copies

- Move the unittest main guard below TestSensitivePasteCopy so running
  the file directly also runs the new tests.
- Add -> None return hints to the new test methods.
- Cover _copy_to_clipboard forwarding the sensitive hint when a tool
  declines and the next one is tried.
- Docs: history suppression needs wl-copy 2.2+ and a clipboard manager
  that honours the hint; older wl-copy and the XWayland xclip/xsel path
  still produce ordinary copies.

* fix(clipboard): type the wl-copy probe cache and adapt the X11 paste test

- Declare _wl_copy_sensitive_supported as a typed class default instead
  of reading it through getattr(), which mypy (now in CI since #968)
  flags as returning Any.
- test_plain_x11_clipboard_paste_uses_xdotool_and_x11_clipboard
  asserted the exact _copy_to_clipboard kwargs. Check the text and the
  X11 tools instead, so it does not depend on the sensitive hint (which
  xclip ignores) or on the copy_to_clipboard setting.

* test(clipboard): wait for the restore thread instead of a fixed sleep

TestSensitivePasteCopy slept 0.5 s and hoped the background restore had
run. On a slow runner the patches could end first. Wait on an event set
by the restore copy, inside the patches, with a generous timeout.

docs/USER_GUIDE.md

@@ -36,6 +36,8 @@ On non-US layouts such as German Neo, that chord uses the key that types **v** o
 
 On X11, **Paste long dictations** opts in to clipboard-pasting dictations of 30 or more characters instead of typing them key by key. It is off by default. Vocalinux restores the previous clipboard contents after pasting unless **Copy to Clipboard** is enabled. If the clipboard contains an image or other non-text content, Vocalinux types the dictation and leaves that content alone.
 
+The clipboard copy behind these pastes is temporary: your previous clipboard comes back right after. When the copy goes through `wl-copy` 2.2 or newer, it is also marked sensitive (`wl-copy --sensitive`), and clipboard managers that honor that hint, such as Klipper or cliphist, keep it out of their history. Older `wl-copy` builds and X11 or XWayland pastes (`xclip`/`xsel`) have no such hint, so there the dictated text can still show up in history. With **Settings → Dictation → Copy to Clipboard** on, the dictated text is an ordinary copy and stays in history.
+
 **Other audio.** Settings → Audio → Other audio can lower speakers and headphones while the microphone is open, then put that volume back. It stays off until you turn it on. Level while dictating is a percent of the current volume (0 is silent). Quitting mid-dictation puts the volume back; if the app crashes first, the next launch does.
 
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