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

RequireSurroundingText asks the client to send its text; it arrives
through SetSurroundingText as an IBusText and a cursor counted in
runes, and DeleteSurroundingText asks for runes back before the commit
that replaces them.  The official libibus client now proves the whole
turn: 한자 typed, the Hanja key, Enter, and 漢字 arrives with the 한
taken away.

A client that has set EffectivePostProcessKeyEvent is never asked for
its text.  It reads the key's commits back after ProcessKeyEvent
returns, and a DeleteSurroundingText signal is not one of the things
that reply can carry, so the deletion would land after the text it was
meant to make room for and eat the wrong runes.

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

tests/ibus_live_test.c

@@ -129,6 +129,7 @@ runcontract(char *address)
 		"SetCapabilities",
 		"SetCursorLocation",
 		"SetCursorLocationRelative",
+		"SetSurroundingText",
 	};
 	DBusConnection *conn;
 	char path[96];