Fix ISO Latin-1 decoder sign extension

[metsw24-max]

Fix sign-extension corruption in ISOLatinCharsetDecoder::decode when decoding bytes >= 0x80.

charsetdecoder.cpp previously converted input bytes using:

static_cast<unsigned int>(*src)

Because char is signed by default on common MSVC/GCC/Clang x86/x64 builds, bytes in the range 0x80..0xFF were sign-extended to 0xFFFFFFxx. This caused Transcoder::encode to treat them as invalid Unicode values, producing U+FFFD on UTF-8 builds (or invalid wchar_t values on WCHAR builds).

The fix casts through unsigned char first, matching the existing unsigned-byte handling patterns already used elsewhere in the codebase.

Impact

properties.cpp selects this decoder for Java .properties parsing, so any Latin-1 characters above ASCII in log4cxx configuration files could be silently corrupted. This affects cases such as:

• accented file paths
• logger names
• layout patterns
• localized configuration text

Changes

• Fix signed-char sign extension in ISOLatinCharsetDecoder::decode
• Add testISOLatinHighBytes
• Verify round-trip behavior for all bytes 0x80..0xFF