patch 9.1.1812: completion: flicker with slow LSPs

Problem:  completion: flicker with slow LSPs
Solution: Disable flushing the changed text (Girish Palya).

In insert-mode completion, the leader text is temporarily deleted while
searching for completion candidates. If the LSP server responds slowly,
the client may call `:sleep` to wait, which triggers `out_flush()`. This
causes the deleted text to briefly disappear before being redrawn when
results arrive, producing a visible flicker.

There are two possible fixes:

1. Suppress flushing while a user function (e.g. LSP client) is waiting.
2. Reinsert the deleted text before invoking the user function.

This Commit implements (1), which is the simpler solution, though somewhat
heavy-handed. If you think this may introduce unwanted side effects, I
can rework it to use (2).

closes: #18439

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/term.c

@@ -2772,7 +2772,7 @@ out_flush(void)
 {
     int	    len;
 
-    if (out_pos == 0)
+    if (no_flush > 0 || out_pos == 0)
 	return;
 
     // set out_pos to 0 before ui_write, to avoid recursiveness