Soft-wrapped autosuggestion to hide right prompt for now

Prior to f417cbc981 (Show soft-wrapped portions in autosuggestions,
2025-12-11), we'd truncate autosuggestions before the right prompt.
We no longer do that for autosuggestions that soft-wrap, which means
we try to draw both right prompt and suggestion in the same space.

Make suggestion paint over right prompt for now, since this seems to
be a simple and robust solution.  We can revisit this later.

Fixes #12255

tests/checks/tmux-right-prompt.fish

@@ -0,0 +1,22 @@
+#RUN: %fish %s
+#REQUIRES: command -v tmux
+
+isolated-tmux-start -C '
+    function fish_right_prompt
+        echo RP
+    end
+    history append "echo $(string repeat 100 a)"
+'
+isolated-tmux send-keys Enter e
+tmux-sleep
+isolated-tmux capture-pane -p
+# CHECK: prompt 0> {{ *}} RP
+# CHECK: prompt 0> echo {{(a{65,})}}
+# CHECK: {{(a{35,})}}
+
+isolated-tmux send-keys C-h C-l "echo line1" M-Enter "e"
+tmux-sleep
+isolated-tmux capture-pane -p
+# CHECK: prompt 0> echo line1 {{ *}} RP
+# CHECK:           echo {{(a{65,})}}
+# CHECK: {{(a{35,})}}