wl: the text before the cursor, and the runes to take back

The surrounding_text event was taken and thrown away.  It carries the
text around the cursor with a byte offset into it, and belongs to the
activation like the content type, so it is pending until done and
starts empty at every activate.  delete_surrounding_text counts bytes
where the engine counts runes, and the frontend holds the text those
runes are in, so it measures them itself.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit_test.c

@@ -149,6 +149,7 @@ static const struct ct_test tests[] = {
 	{ "xim/commit-encoding", xim_adapter_commit_encoding },
 	{ "wl/modifier-mask", wl_modifier_mask },
 	{ "wl/forwarded-keys", wl_forwarded_keys },
+	{ "wl/surrounding-text", wl_surrounding_text },
 	{ "wl/repeat-ends", wl_repeat_ends },
 };
 #else