Support propertyNames enum constraint (OpenAPI 3.1)

Problem

OpenAPI 3.1 allows constraining object property names via propertyNames with enum values. Orval generated the [key: string] index signature instead of a union like [key: 'foo' | 'bar'].

Before:

export interface MyObject {
  [key: string]: string;
}

Solution

• Detect propertyNames with enum values
• Generate a union type for the index signature key

After:

export interface MyObject {
  [key: 'foo' | 'bar']: string;
}

Changes

• Added getPropertyNamesEnum() to extract enum values
• Added getIndexSignatureKey() to format keys
• Updated getObject() index signature generation
• Added tests

Testing

• propertyNames with enum
• Boolean additionalProperties with enum
• Enum with specific types
• Fallback to string when no enum
• Works with existing properties and enum together

All tests pass.

Related

Fixes #2196