BuiltInConverterValidator

Validates the converter configuration for ambiguity at compile-time.

Scope (intentionally narrow):

• Checks the GLOBAL @KMapperConfig(converters=...) list for duplicate pairs, ORIENTATION-NORMALIZED: and are the same pair (converters are pair-keyed and orientation-independent), so flipped duplicates are caught too. Two converters in the global list for the same pair is genuinely ambiguous → ERROR.

• Does NOT scan all MapTypeConverter subclasses in the compilation. A converter referenced ONLY via a per-field @ConvertWith/@ConvertTo/@ConvertFrom use= override is explicit and unambiguous; it must never trigger a duplicate error even when its (S,T) pair collides with a global or built-in converter.