As mentioned in

    commit 368f2af6
    Author: Matthias Clasen <mclasen@redhat.com>
    Date:   Mon Oct 2 08:47:53 2023 -0400

        a11y: Be safe against non-UTF8 text

, the string insertion APIs take string + length
and only insert up to `length` bytes of the
given string.

The AT-SPI "TextChanged" event however
is using a character count, and `emit_text_changed`
also gets called with the character count
along with the string.

However, `g_strndup` used in `emit_text_changed`
so far takes a byte count, not a character count.

Adapt `emit_text_changed` to just use the
passed text as is and make it the responsibility
of the callers to pass only the actually
inserted/removed string.

Most of the callers in `gtk/a11y/gtkatspitext.c`
already did that. Adapt two missing ones to do
likewise.

Fixes: #6151