Preserve extra JSON schema properties in ToolJson serialization

[Copilot]

• Understand the issue: ToolJson class strips extra JSON properties (like $defs/definitions) when converting AI function declarations to OpenAI tool parameters
• Identify the fix: Add JsonExtensionData to preserve unknown properties in ToolJson
• Implement the fix in OpenAIClientExtensions.cs
• Add unit test to verify $defs and other properties are preserved
• Run targeted tests to validate changes (all 74 conversion tests passed on all 3 frameworks)
• Update code comment to reflect new behavior (removed "remove extra properties" wording)
• Run code review (no issues found)
• Run codeql_checker (no issues found)

Original prompt

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This section details on the original issue you should resolve

<issue_title>[API Question]: ToolJson doesn't have definitions object?</issue_title>
<issue_description>

extensions/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIClientExtensions.cs

Line 255 in 140e405

public string Type { get; set; } = "object";

It appears that by the ToolsJson not having a "definitions" object prevents tool calling behavior using Microsoft.Extensions.AI from matching that of using the OpenAI client despite nearly identical setup. This causes problems when using reference types in function declaration signatures when creating a AIFunctionDeclaration via AIFunctionFactory.CreateDeclaration as the definitions for object types are not provided to the model in the prompt, leading to a failure of the model to create a valid / deserializable arguments.

</issue_description>

<agent_instructions>OpenAI accepts other properties on the top-level JSON object without error, both in strict and non-strict mode; we shouldn't be unnecessarily be stripping them off.</agent_instructions>

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

@stephentoub I'm not following. Can you share a minimal repro of the problem you're hitting?

• Fixes [API Question]: ToolJson doesn't have definitions object? #7249

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Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR fixes an issue where ToolJson was stripping extra top-level JSON schema properties (such as $defs and definitions) during serialization roundtrip, causing function calls with reference types to fail.

Changes:

• Added [JsonExtensionData] attribute to ToolJson class to preserve unrecognized schema properties
• Added comprehensive test verifying $defs property preservation through the conversion pipeline

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIClientExtensions.cs	Added ExtensionData property with [JsonExtensionData] attribute to ToolJson class to preserve extra schema properties like $defs and definitions
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIConversionTests.cs	Added test AsOpenAIChatTool_PreservesExtraTopLevelPropertiesLikeDefs that validates $defs property is preserved during function declaration to OpenAI tool conversion