.NET: add FunctionApprovalResponseItemParam to fix unrecognized type discriminator (fixes #6006)

[hanhan761]

Summary

When DevUI sends a human-in-the-loop approval response via the Responses API, the payload includes an item with type "function_approval_response". The ItemParamConverter did not have a case for this type, causing a 400 error.

This fix adds:

1. FunctionApprovalResponseItemParam class in ItemParam.cs — a new ItemParam subclass with request_id and approved properties
2. Case in ItemParamConverter.cs — maps "function_approval_response" to the new type
3. [JsonSerializable] attribute in OpenAIHostingJsonUtilities.cs — registers the type for source-generated JSON serialization

Issue

Fixes #6006

Verification

• All three changes follow existing patterns used by similar item param types (e.g., MCPApprovalResponseItemParam)
• The FunctionApprovalResponseItemParam mirrors the StreamingFunctionApprovalResponded event shape