[3.13] gh-153864: Fix curses window.insch() for non-ASCII characters on a wide build...

[3.13] gh-153864: Fix curses window.insch() for non-ASCII characters on a wide build (GH-153865) (GH-154154)

On a wide build, winsch() does not locale-decode a byte above 127, unlike
waddch(), so insch()/mvinsch() stored the wrong character for a non-ASCII
byte of an 8-bit locale ('€' 0xA4 under ISO-8859-15 became U+00A4 '¤').
Decode the byte with btowc() and insert it as a wide character, as addch()
effectively does.

(cherry picked from commit 223cbffca6df7e4c8c77b360a4e6ba0472255643)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>