Add support for top-level oneOf/union schemas in OpenAPI spec

[tomasweigenast]

Disclaimer

This pull request was fully generated using GitHub Agent. I reviewed all the changes, validated the approach, and tested the implementation with my own examples. No code in this PR was written manually by me; the agent produced all of it. This was my first time using GitHub Agent, and the results were satisfactory based on the tests performed :)

Summary

This PR adds support for parsing and generating Dart/freezed code from top-level oneOf schemas in OpenAPI specifications. Previously, oneOf was only supported when nested inside another schema's property. Check out #58

Problem

The generator did not support this OpenAPI pattern:

"OrderLineChange": {
  "type": "object",
  "oneOf": [
    {"$ref": "#/components/schemas/CreateOrderLine"},
    {"$ref": "#/components/schemas/UpdateOrderLine"},
    {"$ref": "#/components/schemas/DeleteOrderLine"}
  ],
  "discriminator": {
    "propertyName": "type",
    "mapping": {
      "create": "#/components/schemas/CreateOrderLine",
      "update": "#/components/schemas/UpdateOrderLine",
      "delete": "#/components/schemas/DeleteOrderLine"
    }
  }
}

Solution

• Added oneOf and discriminator fields to OpenApiSchemas class
• Modified ModelGenerator to detect top-level oneOf schemas
• Added buildFromTopLevelSchema method in UnionModelStrategy
• Properties from referenced schemas are inlined directly into union constructors
• Static const String key fields are generated and shared across union cases

Generated Output Example

@Freezed(unionKey: r"type")
sealed class OrderLineChange with _$OrderLineChange {
  static const String productIdKey_ = r'product_id';
  static const String quantityKey_ = r'quantity';

  const factory OrderLineChange.create({
    @JsonKey(name: OrderLineChange.productIdKey_) required String productId,
    @JsonKey(name: OrderLineChange.quantityKey_) required int quantity,
  }) = OrderLineChangeCreate;

  const factory OrderLineChange.update({...}) = OrderLineChangeUpdate;
}

Summary

I’m not sure whether this will be merged or if additional work is required (for example, adding tests), but I ran into this issue and needed a solution. I’m sharing it in case it’s useful to others.