feat: support schemas combining 'properties' and 'additionalProperties'

[martin-helmich]

Summary

Schemas that declare both properties and additionalProperties were previously rejected with a GeneratorException ("using 'properties' and 'additionalProperties' in the same schema is currently not supported"). This broke code generation for the mittwald mStudio API, whose spec now contains such a schema (de.mittwald.v1.domainmigration.DomainNotMigratableValidationError's context property).

Such schemas are now generated as regular classes in which all declared properties work as before, while undeclared input properties are collected into an $additionalProperties map:

• buildFromInput() maps undeclared input keys into the map, converting each value according to the additionalProperties schema (including $validate passthrough for object-typed values)
• toJson() serializes the map back out; it is written before the declared properties, so declared properties win on key collisions
• getAdditionalProperties() / withAdditionalProperties() expose the map
• __clone() deep-clones map values where the value type requires it
• object-typed additionalProperties values get their own generated class ({Class}AdditionalPropertiesItem)

Implementation notes

• The invariant check in PropertyBuilder is removed; NestedObjectProperty now claims schemas that have declared properties even when additionalProperties is also present. Purely map-like schemas (only additionalProperties) are still handled by the array property types, so their behavior is unchanged.
• The value type of additionalProperties is modeled as a pseudo-property built via PropertyBuilder, so all existing mapping/serialization/clone expression machinery is reused.
• Schemas that declare a regular property literally named additionalProperties while also using schema-level additionalProperties still throw, since the generated member would collide.
• Two new snapshot fixtures cover the string-valued and object-valued variants.

🤖 Generated with Claude Code