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

[3.15] gh-153864: Fix curses window.insch() for non-ASCII characters on a wide build (GH-153865) (GH-154117)

On a wide build, winsch() does not locale-decode a byte above 127, unlike
waddch(), so insch() inserted '¤' (U+00A4) instead of '€' for byte 0xA4
under ISO-8859-15.  Decode the byte with btowc() and insert it as a wide
character, like addch().

(cherry picked from commit 223cbffca6df7e4c8c77b360a4e6ba0472255643)

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