Add ImageGenerationToolCallContent and ImageGenerationToolResultContent to JSON serialization infrastructure

[Copilot]

ImageGenerationToolCallContent and ImageGenerationToolResultContent were missing from the JSON serialization infrastructure, preventing polymorphic serialization through AIContent references and breaking NativeAOT scenarios.

Changes

• Added type registrations in AIJsonUtilities.Defaults.cs:

  • Called AddAIContentType() for both types with discriminator IDs "imageGenerationToolCall" and "imageGenerationToolResult"
  • Added [JsonSerializable] attributes to JsonContext for source generation support

• Added comprehensive tests:

  • ImageGenerationToolCallContentTests.cs - validates properties and polymorphic serialization
  • ImageGenerationToolResultContentTests.cs - validates properties including nested Outputs list and polymorphic serialization
  • Updated AIContentTests.Serialization_DerivedTypes_Roundtrips to include both types

Example

// Now works correctly - type discriminator ensures proper deserialization
AIContent content = new ImageGenerationToolResultContent 
{ 
    ImageId = "img123",
    Outputs = [new DataContent(imageBytes, "image/png")]
};

string json = JsonSerializer.Serialize(content, AIJsonUtilities.DefaultOptions);
// Emits: {"$type":"imageGenerationToolResult","ImageId":"img123",...}

AIContent deserialized = JsonSerializer.Deserialize<AIContent>(json, AIJsonUtilities.DefaultOptions);
// Correctly reconstructs as ImageGenerationToolResultContent

Follows the same pattern as other experimental content types (CodeInterpreterToolCallContent, McpServerToolCallContent, etc.).

Original prompt

Problem

ImageGenerationToolCallContent and ImageGenerationToolResultContent are missing from AIJsonUtilities.Defaults.cs. Both types are [Experimental] AIContent-derived types (just like CodeInterpreterToolCallContent, McpServerToolCallContent, etc.) but were not added to the JSON serialization infrastructure. This means:

1. No polymorphic type discriminator is emitted when serializing these types as AIContent, so deserialization cannot reconstruct the correct derived type.
2. No source-generated type info exists for NativeAOT scenarios (where JsonSerializer.IsReflectionEnabledByDefault is false), so serialization will fail entirely.

Required Changes

1. src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Defaults.cs

In CreateDefaultOptions() method (around line 61, after the CodeInterpreterToolResultContent line), add:

AddAIContentType(options, typeof(ImageGenerationToolCallContent), typeDiscriminatorId: "imageGenerationToolCall", checkBuiltIn: false);
AddAIContentType(options, typeof(ImageGenerationToolResultContent), typeDiscriminatorId: "imageGenerationToolResult", checkBuiltIn: false);

In the JsonContext source-generation attributes (around line 136, after CodeInterpreterToolResultContent), add:

[JsonSerializable(typeof(ImageGenerationToolCallContent))]
[JsonSerializable(typeof(ImageGenerationToolResultContent))]

These should be placed near the existing // IImageGenerator section or alongside the other experimental content types — follow the existing pattern.

2. Add tests to validate polymorphic serialization round-tripping

There is an existing test file for AIJsonUtilities serialization tests. Add tests that validate ImageGenerationToolCallContent and ImageGenerationToolResultContent can be serialized and deserialized polymorphically through AIContent references, similar to how other experimental content types are tested.

Look at how other AIContent-derived types (especially experimental ones like CodeInterpreterToolCallContent, CodeInterpreterToolResultContent, McpServerToolCallContent, etc.) are tested for polymorphic JSON round-tripping in the test suite, and follow the same pattern for the two image generation content types.

For ImageGenerationToolCallContent, ensure the ImageId property round-trips.
For ImageGenerationToolResultContent, ensure both the ImageId and Outputs properties round-trip (including nested AIContent items in the Outputs list).

Context

• ImageGenerationToolCallContent is defined in src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/ImageGenerationToolCallContent.cs
• ImageGenerationToolResultContent is defined in src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/ImageGenerationToolResultContent.cs
• Both are marked [Experimental(DiagnosticIds.Experiments.AIImageGeneration)]
• The fix follows the exact same pattern used for CodeInterpreterToolCallContent/CodeInterpreterToolResultContent and the MCP content types

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Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Adds missing JSON polymorphic/source-gen registrations for image generation tool content types so they can round-trip through AIContent and work in NativeAOT scenarios.

Changes:

• Registered ImageGenerationToolCallContent / ImageGenerationToolResultContent in AIJsonUtilities.DefaultOptions polymorphic configuration with type discriminators.
• Added [JsonSerializable] entries for both types to the source-generated JsonContext.
• Added targeted unit tests and expanded the existing derived-type roundtrip test set.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Defaults.cs	Registers the two image generation tool content types for polymorphic serialization and source generation.
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/ImageGenerationToolCallContentTests.cs	Verifies default/property behavior and polymorphic JSON roundtrip via AIContent.
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/ImageGenerationToolResultContentTests.cs	Verifies Outputs behavior and both concrete + polymorphic JSON roundtrips.
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/AIContentTests.cs	Extends derived-type serialization roundtrip coverage to include the new types.

[ericstj]

LGTM. Thanks for adding more tests to cover these.