fix: Use custom JsonConverter for A2AEvent and A2AResponse instead of [JsonPolymorphic] to fix discriminator deserialization issues

[Blackhex]

Summary

• Replace [JsonPolymorphic] with custom JsonConverter implementations for A2AEvent and A2AResponse to fix discriminator-based deserialization.
• Ensure consistent round-trip serialization using source-generated JsonContext and explicit kind discriminators.

Problem

• [JsonPolymorphic] produced inconsistent discriminator deserialization across contexts/AOT, causing events to fail to materialize to concrete types or lose the kind field.

Changes

• Add A2AEventConverter and A2AResponseConverter with strict kind handling:
  • Read: parse once, route by kind → message, task, status-update, artifact-update; throw on unknown/missing.
  • Write: serialize via A2AJsonUtilities.JsonContext to concrete metadata.
• Add explicit Kind overrides:
  • Message → message, AgentTask → task, TaskStatusUpdateEvent → status-update, TaskArtifactUpdateEvent → artifact-update.
• Expand source-gen coverage in A2AJsonUtilities.JsonContext for affected types.
• Add comprehensive unit tests in tests/A2A.UnitTests/Models/A2AResponseTests.cs.

Testing

• Deserialization as A2AEvent: succeeds for all four kinds; throws for unknown/missing kind.
• Deserialization as A2AResponse: succeeds for message/task; throws for status-update/artifact-update.
• Serialization as A2AEvent/A2AResponse: kind is always present and correct; selected exact-shape assertions for known cases.
• dotnet test A2A.slnx -v m passes on net8.0/net9.0.

Impact

• Public model shape remains stable; behavior is stricter and more predictable.
• Potentially breaking only if external consumers subclass A2AEvent/TaskUpdateEvent: they must implement Kind, and custom kind values are not yet routed by the new converters.

Migration Notes

• If you have custom event types, add public override string Kind => "<your-kind>"; and handle custom routing via an app-level converter or open an issue to extend built-in routing.

Files

• Updated: src/A2A/Models/A2AResponse.cs, src/A2A/Models/Message.cs, src/A2A/Models/AgentTask.cs, src/A2A/Models/TaskStatusUpdateEvent.cs, src/A2A/Models/TaskArtifactUpdateEvent.cs, src/A2A/A2AJsonUtilities.cs
• Added tests: tests/A2A.UnitTests/Models/A2AResponseTests.cs

Related

• Fixes Serializing Message property of MessageSendParameters does not include "kind" #138 .

[Copilot]

Pull Request Overview

This PR replaces the [JsonPolymorphic] attribute with custom JsonConverter implementations for A2AEvent and A2AResponse to fix discriminator-based deserialization issues in AOT scenarios.

Key changes:

• Custom converters with explicit kind discriminator routing for consistent serialization/deserialization
• Added explicit Kind property overrides to all concrete event types
• Enhanced source-generated JSON context coverage for affected types

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/A2A/Models/A2AResponse.cs	Replaced [JsonPolymorphic] with custom converters and added abstract Kind property
src/A2A/Models/Message.cs	Added explicit Kind property override returning "message"
src/A2A/Models/AgentTask.cs	Added explicit Kind property override returning "task"
src/A2A/Models/TaskStatusUpdateEvent.cs	Added explicit Kind property override returning "status-update"
src/A2A/Models/TaskArtifactUpdateEvent.cs	Added explicit Kind property override returning "artifact-update"
src/A2A/A2AJsonUtilities.cs	Added missing type registrations for source-generated JSON context
tests/A2A.UnitTests/Models/A2AResponseTests.cs	Added comprehensive test coverage for new converter behavior

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[Copilot]

The Read method is being called on a ref Utf8JsonReader that has already been consumed by the previous call. This will cause the reader to be positioned incorrectly. You need to reset the reader position or pass a copy of the reader state.

Suggested change

	throw new JsonException("JSON did not represent an A2AResponse instance.");
	if (reader.TokenType != JsonTokenType.StartObject)
	{
	throw new JsonException("Expected StartObject token");
	}
	using var document = JsonDocument.ParseValue(ref reader);
	var a2aResponse = document.RootElement.Deserialize(A2AJsonUtilities.JsonContext.Default.A2AResponse);
	if (a2aResponse is null)
	{
	throw new JsonException("JSON did not represent an A2AResponse instance.");
	}
	return a2aResponse;

[Blackhex]

Correct me if I am wrong but this comment seems false-positive to me.

[Blackhex]

Closing in favor of #163