Fix OpenSSL paths for restricted Windows code pages

[MadMaxMangos]

Summary

• use API-valid flags and null default-character pointers when the Windows active code page is UTF-8 or GB18030
• preserve best-fit rejection for other active code pages
• reject malformed UTF-16 rather than silently substituting replacement characters

This is a focused follow-up to #470. No new tests or database changes are included.

Verification

• Release mangos_tests target built
• Release realmd target built
• existing OpenSSL tests passed (2/164 selected, 0 failed)
• direct Win32 probes confirmed valid input converts and lone surrogates fail with ERROR_NO_UNICODE_TRANSLATION for code pages 65001 and 54936
• Devin SWE-1.7 focused re-review: APPROVE

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This change is 

[codacy-production]

Up to standards ✅

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