Add support for response summary in OpenAPI 3.2.0

[Copilot]

This PR implements support for the new summary field in response objects as defined in OpenAPI Specification 3.2.0.

Changes Made

Core Implementation

• Added Summary property to OpenApiResponse class implementing IOpenApiSummarizedElement
• Updated IOpenApiResponse interface to derive from IOpenApiSummarizedElement making Summary part of the interface contract
• Updated OpenApiResponseReference to properly delegate the Summary property to its target
• Modified response copy constructors to handle the new Summary field

Version-Specific Serialization

The implementation follows the specification requirements for backward compatibility:

OpenAPI 3.2+: Summary is serialized as a standard field

{
  "summary": "Successful response",
  "description": "A detailed description"
}

OpenAPI 3.1 and earlier: Summary is serialized as an extension field using the x-oai- prefix

{
  "description": "A detailed description", 
  "x-oai-summary": "Successful response"
}

Deserialization Support

• V3.2 deserializer: Handles direct "summary" field parsing
• V3.1 & V3.0 deserializers: Handles "x-oai-summary" extension field parsing with fallback to regular extensions

Testing

• Added comprehensive serialization tests for all OpenAPI versions
• Added deserialization tests with sample YAML documents
• Updated existing reference tests to handle the new Summary property
• All 67 existing response-related tests continue to pass

Usage Example

// Summary is now part of the IOpenApiResponse interface contract
IOpenApiResponse response = new OpenApiResponse 
{
    Summary = "Operation completed successfully",
    Description = "The requested operation has been completed",
    Content = new Dictionary<string, OpenApiMediaType>
    {
        ["application/json"] = new OpenApiMediaType 
        { 
            Schema = new OpenApiSchema { Type = JsonSchemaType.Object } 
        }
    }
};

// Access Summary directly through the interface
Console.WriteLine($"Summary: {response.Summary}");

// Works with all OpenAPI versions with appropriate serialization
var v32Json = await response.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_2);
var v31Json = await response.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_1);

This implementation maintains full backward compatibility while enabling the new summary functionality for OpenAPI 3.2.0 documents. The interface hierarchy now properly expresses that all response objects have a Summary property.

Fixes #17

Original prompt

---

This section details on the original issue you should resolve

<issue_title>add support for response summary</issue_title>
<issue_description>OAS 3.2.0 defines a new summary field

We should add the new field to the model, implement serialization/deserialization for it.

When serializing to OAS 3.10, the property should be using an x-oai- prefix to make it an extension</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #17

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[baywet]

@copilot also update the IOpenAPIResponse interface to derive from ISummarizedElement

[Copilot]

@copilot also update the IOpenAPIResponse interface to derive from ISummarizedElement

Updated the IOpenApiResponse interface to derive from IOpenApiSummarizedElement. This makes the Summary property part of the interface contract for all response objects. The implementation has been simplified accordingly, and all tests continue to pass. (commit 8f5c5cb)