wchar_compat: Implement WideCharToMultiByte for non-Windows platforms

[seer-by-sentry]

Fixes CLIENT-S3. The issue was that: Unsafe wcstombs macro implementation for WideCharToMultiByte causes Access Violation when processing malformed wide character strings from d3d8.dll.

• Implements WideCharToMultiByte function for non-Windows platforms using wcstombs.
• Includes null pointer checks for input wcstr.
• Handles cases where cch is -1 (null-terminated string) or 0.
• Implements buffer size validation to prevent overflows.
• Ensures null termination of the output mbstr within the provided buffer size.
• Returns the number of bytes written to mbstr.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 2501785

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