Harden MappingConfiguration generator for real-world type scenarios

[davidkallesen]

Summary

This PR significantly improves the MappingConfiguration and Object Mapping generators to handle real-world type
scenarios encountered when mapping between generated API types (from Atc.Rest.Api.SourceGenerator) and domain
types. It fixes multiple issues with positional records, struct handling, nullable types, and metadata-loaded
types, while adding new built-in type conversions and auto-generated nested mapping sub-methods.

Changes

✨ Features

• Add ATCMCF014 diagnostic warning when types cannot be resolved (sibling source generator limitation)
• Support partial constructor matching for positional records with default values
• Add string ↔ Uri built-in type conversions with nullable safety
• Add string ↔ enum conversions using Enum.TryParse
• Auto-generate private mapping sub-methods for nested and collection element types not declared as top-level
  mappings
• Support nullable type unwrapping (Nullable<T>) across all type analysis
• Add underscore-normalized enum value matching (Block_DE_L2 → BlockDEL2)

🐛 Fixes

• Fix null-conditional operator (?.) on value type nested mappings — use direct call for struct sources
• Fix ref-to-struct nested mapping — use ternary pattern instead of ?.MapToX()! which produces T?
• Fix null guard (if (source is null)) emitted for struct input parameters
• Fix [MapConfigProperty] generating object initializer instead of constructor for positional records with
  renamed properties
• Fix numeric type conversions (uint → int, etc.) rejected in constructor matching
• Fix collection type compatibility (IList<A> → List<B>) rejected in constructor matching
• Fix positional records with collection enum properties falling back to object initializer
• Fix private-setter properties invisible from metadata-loaded types (compiled assembly references)
• Fix built-in type conversions incorrectly detected as nested mappings

♻️ Refactoring

• Extract UnwrapNullable and IsNullableValueType helpers into shared MappingTypeAnalyzer
• Add SourceCollectionElementType to PropertyMapping record for same-element-type detection
• Add ConstructorDefaultParameter record for partial constructor matching with defaults

Notes

• The private-setter fix resolves the root cause of the previously unreproducible "int params with defaults
  skipped" issue — the real problem was Roslyn hiding SetMethod on metadata-loaded types
• Test infrastructure now includes CompileToAssembly and GetGeneratedOutputWithAssemblyReference helpers for
  testing metadata-vs-source symbol behavior