This enforces explicit provider specification.
Automatic fallback to preferences or "openai" is removed.
Users must now pass the provider name explicitly.
default provider functionality was improperly removed and implemented (some parts still remain intact), leading to UAF.
Add helper functions to parse hex digits and encode UTF-8 characters. Update buffer allocation to account for UTF-8 expansion and implement full surrogate pair support for characters outside the BMP. Previously, \uXXXX sequences were passed through verbatim; they are now correctly decoded into proper UTF-8 strings.
