Add support for itemEncoding and prefixEncoding fields in OpenApiMediaType

[Copilot]

Summary

This PR implements support for the itemEncoding and prefixEncoding fields in OpenApiMediaType as defined in the OpenAPI 3.2.0 specification.

Changes

New Properties

• itemEncoding (OpenApiEncoding?): An encoding object for items in an array schema
• prefixEncoding (IList<OpenApiEncoding>?): An array of encoding objects for prefixItems in an array schema

Version-Specific Serialization

Following the pattern established for other 3.2.0 fields:

OpenAPI 3.2.0+: Fields are serialized as native properties

{
  "itemEncoding": {
    "contentType": "text/plain"
  },
  "prefixEncoding": [
    {
      "contentType": "application/json"
    }
  ]
}

OpenAPI 3.1 and earlier: Fields are serialized as x-oai- prefixed extensions

{
  "x-oai-itemEncoding": {
    "contentType": "text/plain"
  },
  "x-oai-prefixEncoding": [
    {
      "contentType": "application/json"
    }
  ]
}

Implementation Details

1. Constants: Added ItemEncoding and PrefixEncoding to OpenApiConstants
2. Model: Added properties to OpenApiMediaType with proper copy constructor support
3. Serialization: Updated SerializeInternal to handle version-specific output
4. Deserialization: Updated all version deserializers (v3.0, v3.1, v3.2) to read the fields
5. Tests: Added comprehensive serialization tests for both v3.2 (native) and v3.1 (extension) formats
6. Public API: Updated PublicApi.approved.txt to reflect the new public surface

Testing

All existing tests pass, plus 4 new tests covering:

• itemEncoding serialization in v3.2 (native)
• itemEncoding serialization in v3.1 (extension)
• prefixEncoding serialization in v3.2 (native)
• prefixEncoding serialization in v3.1 (extension)

Fixes

Resolves the issue by providing full support for these OAI 3.2.0 media type fields with backward compatibility for earlier versions.

Warning

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>add support for the mediaType itemEncoding and prefixEncoding fields</issue_title>
<issue_description>OAI 3.2.0 defines two new fields for media type.

• itemEncoding (OpenAPIEncoding?)
• prefixEncoding (OpenAPIEncoding[]?)

In both cases the fields need to be added to the model and serialization/deserialization should be updated.

When serializing to OAI 3.1.0 or lower, those properties should be prefixed with x-oai- so they are stored as extensions</issue_description>

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

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

Fixes #10

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