feat: Map JSON DataContent to Part.Data

[SergeyMenshykh]

Summary

• map JSON DataContent payloads to structured Part.Data values
• recognize application/json, media type parameters, and +json suffixes
• preserve the original media type and surface invalid JSON
• keep non-JSON DataContent mapped to Part.Raw

Tests

• dotnet test tests\A2A.UnitTests\A2A.UnitTests.csproj --no-restore --filter "FullyQualifiedName~AIContentExtensionsTests" --verbosity quiet

Fixes #447

[chopmob-cloud]

Verified this independently against current main (built net8.0 and net10.0, ran the round trip). The forward direction here is correct: JSON DataContent maps to Part.Data, the +json structured suffix and media type parameters are recognised, the original media type is preserved onto the part, and invalid JSON declared as JSON now surfaces a JsonException. All of that matches what #447 asked for, and the added tests cover it.

One observation on the "symmetric round-tripping" motivation from #447. The symmetry currently holds only for exactly application/json. The reverse leg, ToAIContent's part.Data branch, is unchanged and hardcodes "application/json", ignoring part.MediaType. So the media type this PR carefully preserves on the way out is dropped on the way back:

in=application/json                | forwardPart=application/json                | reverse=application/json
in=application/problem+json        | forwardPart=application/problem+json        | reverse=application/json
in=application/vnd.acme.thing+json | forwardPart=application/vnd.acme.thing+json | reverse=application/json

Media type parameters and casing are lost the same way (application/json; charset=utf-8 comes back as bare application/json).

A small change on the reverse branch closes the loop for the +json types the forward direction now supports:

content = new DataContent(
    JsonSerializer.SerializeToUtf8Bytes(data, A2AJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonElement))),
    IsJsonMediaType(part.MediaType) ? part.MediaType! : "application/json");

Guarding on IsJsonMediaType (rather than a bare part.MediaType ?? "application/json") keeps the invariant that JSON bytes are never labelled with a non-JSON media type. With this, application/problem+json and application/vnd.acme.thing+json round trip losslessly, and all existing AIContentExtensionsTests stay green on both target frameworks. Happy to open a follow up if that is useful, or it folds naturally into this PR.

[SergeyMenshykh]

Verified this independently against current main (built net8.0 and net10.0, ran the round trip). The forward direction here is correct: JSON DataContent maps to Part.Data, the +json structured suffix and media type parameters are recognised, the original media type is preserved onto the part, and invalid JSON declared as JSON now surfaces a JsonException. All of that matches what #447 asked for, and the added tests cover it.

One observation on the "symmetric round-tripping" motivation from #447. The symmetry currently holds only for exactly application/json. The reverse leg, ToAIContent's part.Data branch, is unchanged and hardcodes "application/json", ignoring part.MediaType. So the media type this PR carefully preserves on the way out is dropped on the way back:

in=application/json                | forwardPart=application/json                | reverse=application/json
in=application/problem+json        | forwardPart=application/problem+json        | reverse=application/json
in=application/vnd.acme.thing+json | forwardPart=application/vnd.acme.thing+json | reverse=application/json

Media type parameters and casing are lost the same way (application/json; charset=utf-8 comes back as bare application/json).

A small change on the reverse branch closes the loop for the +json types the forward direction now supports:

content = new DataContent(
    JsonSerializer.SerializeToUtf8Bytes(data, A2AJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonElement))),
    IsJsonMediaType(part.MediaType) ? part.MediaType! : "application/json");

Guarding on IsJsonMediaType (rather than a bare part.MediaType ?? "application/json") keeps the invariant that JSON bytes are never labelled with a non-JSON media type. With this, application/problem+json and application/vnd.acme.thing+json round trip losslessly, and all existing AIContentExtensionsTests stay green on both target frameworks. Happy to open a follow up if that is useful, or it folds naturally into this PR.

Thanks for catching this. It makes sense, and I’ve added the suggested change and tests.