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 and deserialization tests for both v3.2 (native) and v3.1/v3.0 (extension) formats
6. Public API: Updated PublicApi.approved.txt to reflect the new public surface
7. Bug Fix: Fixed typo in OpenApiEncodingDeserializer where allowedReserved was misspelled (should be allowReserved)

Testing

All existing tests pass, plus 10 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)
• itemEncoding deserialization in v3.2 (native)
• itemEncoding deserialization in v3.1 (x-oai- extension)
• itemEncoding deserialization in v3.0 (x-oai- extension)
• prefixEncoding deserialization in v3.2 (native)
• prefixEncoding deserialization in v3.1 (x-oai- extension)
• prefixEncoding deserialization in v3.0 (x-oai- 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

example.org926f4f21-88e7-4dc5-bf88-6c529bb77844

• Triggering command: /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/work/OpenAPI.NET/OpenAPI.NET/test/Microsoft.OpenApi.Hidi.Tests/bin/Debug/net8.0/Microsoft.OpenApi.Hidi.Tests.runtimeconfig.json --depsfile /home/REDACTED/work/OpenAPI.NET/OpenAPI.NET/test/Microsoft.OpenApi.Hidi.Tests/bin/Debug/net8.0/Microsoft.OpenApi.Hidi.Tests.deps.json /home/REDACTED/work/OpenAPI.NET/OpenAPI.NET/test/Microsoft.OpenApi.Hidi.Tests/bin/Debug/net8.0/testhost.dll --port 33553 --endpoint 127.0.0.1:033553 --role client --parentprocessid 6047 --telemetryoptedin false (dns block)
• Triggering command: /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/work/OpenAPI.NET/OpenAPI.NET/test/Microsoft.OpenApi.Hidi.Tests/bin/Debug/net8.0/Microsoft.OpenApi.Hidi.Tests.runtimeconfig.json --depsfile /home/REDACTED/work/OpenAPI.NET/OpenAPI.NET/test/Microsoft.OpenApi.Hidi.Tests/bin/Debug/net8.0/Microsoft.OpenApi.Hidi.Tests.deps.json /home/REDACTED/work/OpenAPI.NET/OpenAPI.NET/test/Microsoft.OpenApi.Hidi.Tests/bin/Debug/net8.0/testhost.dll --port 38053 --endpoint 127.0.0.1:038053 --role client --parentprocessid 11070 --telemetryoptedin false (dns block)

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Fixes #10

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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[baywet]

@copilot add unit tests for deserialization

[Copilot]

@copilot add unit tests for deserialization

Added comprehensive deserialization tests for itemEncoding and prefixEncoding fields in commit b4eec89. The tests cover:

• V3.2: Native itemEncoding and prefixEncoding field deserialization
• V3.1 & V3.0: x-oai-itemEncoding and x-oai-prefixEncoding extension deserialization

Also fixed a bug in the OpenApiEncodingDeserializer where allowedReserved was misspelled (should be allowReserved), which was preventing proper deserialization of the allowReserved field.