Ensure all ResponseItems are yielded in AIContent

[crickman]

Enable the following logic to handle cases for processing streaming updates without an AIContent abstraction (RawRepresentation only):

In the absence of this additional logic, the following conversion results in an empty ChatMessage:

• https://github.com/dotnet/extensions/blob/main/src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponseExtensions.cs#L136
  -https://github.com/dotnet/extensions/blob/main/src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponseExtensions.cs#L485

Microsoft Reviewers: Open in CodeFlow

[crickman]

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Pull request overview

This PR adds a generic fallback handler for streaming response items that don't have dedicated AIContent abstractions, addressing the issue where certain streaming updates would result in empty ChatMessage objects. The solution introduces a HashSet to track types with explicit handlers and provides a catch-all case to wrap unhandled items in AIContent with RawRepresentation.

Key changes:

• Adds _responseItemTypesWithHandlers HashSet to track response item types with dedicated handlers
• Removes the specific case for McpToolDefinitionListItem (line 419-422)
• Adds a generic handler for StreamingResponseOutputItemDoneUpdate that creates AIContent with RawRepresentation for types not explicitly handled elsewhere