Fix foundry agents connect serialization bug

[JasonYeMSFT]

What does this PR do?

When the agent run steps include file search tool call result, Azure MCP creates an annonymous object that is not serializable and cause the failure. The fix is to create staticly typed objects instead and register this static type in the Json serialization context. I replaced the function that accepts aribtrary object argument with a few overloads that accept known serializable types to prevent us from passing non-serialzable objects to it.

GitHub issue number?

#1170

Pre-merge Checklist

• Required for All PRs
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  • Added comprehensive tests for new/modified functionality
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  • Validate README.md changes using script at eng/scripts/Process-PackageReadMe.ps1. See Package README
  • Updated command list in /servers/Azure.Mcp.Server/docs/azmcp-commands.md and/or /docs/fabric-commands.md
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• Extra steps for Azure MCP Server tool changes:
  • Updated test prompts in /servers/Azure.Mcp.Server/docs/e2eTestPrompts.md
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[Copilot]

Pull Request Overview

This PR fixes a serialization bug in the Foundry agents connect functionality. When agent run steps include file search tool call results, Azure MCP was creating anonymous objects that couldn't be serialized. The fix introduces statically typed AgentFileSearchResult model class and replaces overloaded methods that accepted arbitrary object parameters with type-safe overloads accepting known serializable types (string, JsonElement, IDictionary<string, string?>).

Key changes:

• Created AgentFileSearchResult and AgentFunctionCallContent model classes with proper JSON property attributes
• Refactored CreateRequestMessage and CreateResponseMessage methods from local functions to static methods with type-safe overloads
• Registered List<AgentFileSearchResult> in the JSON serialization context for AOT compatibility

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
tools/Azure.Mcp.Tools.Foundry/src/Services/FoundryService.cs	Refactored message creation from local functions to static methods with multiple type-safe overloads; converted anonymous objects to statically typed AgentFileSearchResult instances
tools/Azure.Mcp.Tools.Foundry/src/Models/AgentFunctionCallContent.cs	Added new model class for function call content with JSON serialization attributes (currently unused)
tools/Azure.Mcp.Tools.Foundry/src/Models/AgentFileSearchResult.cs	Added new model class to represent file search results with proper JSON property names for serialization
tools/Azure.Mcp.Tools.Foundry/src/Commands/FoundryJsonContext.cs	Registered List<AgentFileSearchResult> type in JSON source generation context for AOT serialization support

[Copilot]

The AgentFileSearchResult type should be added to the JSON serialization context. While List<AgentFileSearchResult> is registered on line 67, the individual type needs to be registered as well for proper AOT serialization. Add [JsonSerializable(typeof(AgentFileSearchResult))] before line 67.

Suggested change

	[JsonSerializable(typeof(ThreadGetMessagesResult))]
	[JsonSerializable(typeof(ThreadGetMessagesResult))]
	[JsonSerializable(typeof(AgentFileSearchResult))]

[JasonYeMSFT]

Is Copilot right? I don't think it's necessary.

[alzimmermsft]

Do we need all these different static methods given FunctionResultContent's constructor is string callId, object? result). Cant we just have:

Suggested change

	internal static Microsoft.Extensions.AI.ChatMessage CreateResponseMessage(string toolCallId, JsonElement result, RunStep step)
	internal static Microsoft.Extensions.AI.ChatMessage CreateResponseMessage(string toolCallId, object? result, RunStep step)