Add support for mediaType itemSchema property (OAS 3.2.0)

[Copilot]

✅ Successfully implemented support for the mediaType itemSchema property (OAI 3.2.0)

Summary of Changes

Core Implementation

• ✅ Added IOpenApiSchema? ItemSchema property to OpenApiMediaType class
• ✅ Added ItemSchema = "itemSchema" constant to OpenApiConstants
• ✅ Updated SerializeInternal method to:
  • Write native itemSchema property for OpenAPI 3.2.0+
  • Write x-oai-itemSchema extension for OpenAPI 3.1.0 and 3.0.0 (backward compatibility)
• ✅ Updated copy constructor to clone ItemSchema property

Deserialization Support

• ✅ Updated V32 deserializer to read native itemSchema property
• ✅ Updated V31 deserializer to read x-oai-itemSchema extension and map to ItemSchema property
• ✅ Updated V3 deserializer to read x-oai-itemSchema extension and map to ItemSchema property

Testing

• ✅ Added serialization tests for all OpenAPI versions (3.0, 3.1, 3.2)
• ✅ Added deserialization tests for V31 and V32
• ✅ Added copy constructor test for ItemSchema property
• ✅ Created sample YAML files for test cases
• ✅ Updated PublicApi.approved.txt to reflect new public API surface
• ✅ Replaced FluentAssertions with Assert.Equivalent in reader tests
• ✅ Using JsonNode.DeepEquals for JSON comparison in serialization tests

Test Results

• ✅ All 908 tests passing in Microsoft.OpenApi.Tests
• ✅ All 348 tests passing in Microsoft.OpenApi.Readers.Tests
• ✅ All 54 tests passing in Microsoft.OpenApi.Hidi.Tests
• ✅ Total: 1,310 tests passing, 0 failing

Code Review Feedback Addressed

• ✅ Use WriteOptionalObject for x-oai-itemSchema serialization
• ✅ Replace Should().BeEquivalentTo() with Assert.Equivalent() in V31 and V32 tests
• ✅ Replace Assert.Equal() with JsonNode.DeepEquals() for JSON comparison

Files Modified (11 files)

• src/Microsoft.OpenApi/Models/OpenApiConstants.cs - Added constant
• src/Microsoft.OpenApi/Models/OpenApiMediaType.cs - Added property and serialization logic
• src/Microsoft.OpenApi/Reader/V3/OpenApiMediaTypeDeserializer.cs - Added deserialization
• src/Microsoft.OpenApi/Reader/V31/OpenApiMediaTypeDeserializer.cs - Added deserialization
• src/Microsoft.OpenApi/Reader/V32/OpenApiMediaTypeDeserializer.cs - Added deserialization
• test/Microsoft.OpenApi.Tests/Models/OpenApiMediaTypeTests.cs - Added tests
• test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt - Updated public API
• test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiMediaTypeTests.cs - Added test
• test/Microsoft.OpenApi.Readers.Tests/V31Tests/Samples/OpenApiMediaType/mediaTypeWithXOaiItemSchema.yaml - Sample
• test/Microsoft.OpenApi.Readers.Tests/V32Tests/OpenApiMediaTypeTests.cs - Added test
• test/Microsoft.OpenApi.Readers.Tests/V32Tests/Samples/OpenApiMediaType/mediaTypeWithItemSchema.yaml - Sample

Implementation follows OAS 3.2.0 specification and maintains backward compatibility with earlier versions.

Original prompt

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This section details on the original issue you should resolve

<issue_title>add support for the mediaType itemSchema property</issue_title>
<issue_description>OAI 3.2.0 defines a new itemSchema property on the media type object.

This property is of type IOpenAPISchema? and serialization/deserialization should be updates to account for this new field.

When serializing to version 3.1.0, the property name should be prefixed with x-oai- so it's treated as an extension.</issue_description>

<agent_instructions>make sure you update the public api export</agent_instructions>

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

Fixes #9

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