validate
Validates the global converter list from @KMapperConfig(converters=...).
Receives pre-extracted (sourceFqn to targetFqn) → converterFqn entries (built by MappingProcessor.discoverCustomConverters before calling this). Reports an error if the same (S,T) pair appears more than once.
Return
true if validation passed, false if errors were found
Parameters
globalConverterEntries
list of ((sourceFqn, targetFqn), converterFqn) entries discovered from all @KMapperConfig annotations in the compilation.