Extend template models with native oneOf/anyOf/allOf support and discriminator CodeType mapping

[Copilot]

The template models had no structural representation of OpenAPI polymorphic schemas (oneOf/anyOf/allOf). The workaround was a simplify-polymorphic-schemas patch that flattened everything into a single object, discarding discriminator info and making it impossible to generate correct language-level union/inheritance constructs (e.g. Jackson @JsonSubTypes).

Model changes

DiscriminatorModel (new type)

• PropertyName string — the JSON property carrying the type tag
• Mapping map[string]CodeType — discriminator value → fully resolved CodeType (name, qualified type, import path, etc.), giving templates access to all type metadata rather than a bare class name string

Model

• OneOf/AnyOf/AllOf []CodeType — resolved variant/parent type references (replaces unused []Model)
• IsOneOf / IsAnyOf / IsAllOf bool — flags for template branching
• Discriminator *DiscriminatorModel — populated when a discriminator is present

Property

• Added Required, ReadOnly, WriteOnly bool from the source schema

BuildComponentModels rewrite

Each schema now follows a dedicated branch instead of collapsing into a flat merge:

• oneOf / anyOf — each *SchemaProxy resolved to a CodeType; discriminator mapping resolved to CodeType values via the component schema registry
• allOf — referenced sub-schemas → AllOf []CodeType parents; inline sub-schemas → properties merged directly (dedup-aware, inheriting required from both the root schema and the inline sub-schema)
• object / array / fallback — unchanged, only reached when no polymorphic case applies

Extracted helpers: buildModelProperties (sets Required/ReadOnly/WriteOnly, handles deduplication across allOf merges) and buildDiscriminatorModel (resolves $ref mapping entries to full CodeType values).

Default patches

Removed simplify-polymorphic-schemas from DefaultCodeGenerationPatches — it is no longer needed as a default; it remains available for opt-in use.

Example — discriminator-based oneOf

Pet:
  oneOf:
    - $ref: '#/components/schemas/Cat'
    - $ref: '#/components/schemas/Dog'
  discriminator:
    propertyName: petType
    mapping:
      cat: '#/components/schemas/Cat'
      dog: '#/components/schemas/Dog'

Produces a PetDto model with:

IsOneOf:       true
OneOf:         [CodeType{Name:"CatDto"}, CodeType{Name:"DogDto"}]
Discriminator: {
  PropertyName: "petType",
  Mapping: {
    "cat": CodeType{Name:"CatDto", QualifiedType:"CatDto", ...},
    "dog": CodeType{Name:"DogDto", QualifiedType:"DogDto", ...},
  }
}

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