Fix file URI handling for Shift_JIS filenames on Windows - #1044

Summary

This pull request improves file path handling in Xdebug on Windows by ensuring proper conversion of Shift_JIS-encoded filenames to UTF-8.
Previously, Shift_JIS-encoded paths could not be correctly represented as file URIs, which caused debugger connection failures and incorrect file references when working with non-UTF-8 environments (e.g., PHP source files in Shift_JIS).

Background

On Windows systems using Shift_JIS locale, __FILE__ and related filenames in PHP source code are encoded in Shift_JIS.
Xdebug expected these to be UTF-8, leading to garbled paths or invalid URIs (e.g., during DBGp communication).

This patch adds encoding detection and proper conversion before generating file URIs.

Changes

1. src/base/base.c
   Added UTF-8 filename preparation for Windows builds:

#if PHP_WIN32
	if (op_array->filename) {
		char *utf8_filename = xdebug_prepare_filename(ZSTR_VAL(op_array->filename));
		if (utf8_filename) {
			xdebug_lowercase_drive_letter(utf8_filename);
			char *utf8_filename_url = xdebug_path_to_url(/sitelet?url=https%3A%2F%2Fgithub.com%2Fxdebug%2Fxdebug%2Fpull%2Futf8_filename%2C%2520strlen%28utf8_filename));
			if (utf8_filename_url) {
				op_array->filename = zend_string_init(utf8_filename_url, strlen(utf8_filename_url), 0);
				free(utf8_filename_url);
			}
			free(utf8_filename);
		}
	}
#endif

2. src/debugger/handler_dbgp.c
   Replaced manual URL encoding with:

xdstrdup(XDEBUG_STR_VAL(XXX))

3. src/lib/usefulstuff.c
   Added new helper functions:

• xdebug_is_valid_utf8(const char *str)
• xdebug_is_valid_shiftjis(const char *str)
• xdebug_is_valid_encoding(const char *str)
  → Detects UTF-8, Shift_JIS, or Unknown
• xdebug_shiftjis_to_utf8(const char *sjis_str)
  → Converts Shift_JIS string to UTF-8 using MultiByteToWideChar / WideCharToMultiByte
• xdebug_prepare_filename(const char *filename)
  → Converts filename to UTF-8 if necessary
• xdebug_lowercase_drive_letter(char *fileuri)
  → Normalizes drive letter casing in file URIs

Impact

• Correct handling of Shift_JIS-encoded PHP source filenames on Windows
• Prevents invalid file URIs in DBGp communication
• No impact on non-Windows or UTF-8 environments (guarded with #if PHP_WIN32)

Testing

• Verified Shift_JIS paths on Windows
• Confirmed proper conversion and URI generation

Credits

Authored by @chimorium
Discussed and refined with ChatGPT (context: Shift_JIS path issue in Xdebug)