.NET: reformat event sequence in OpenAI Responses hosting

[DeagleGross]

Reformatted event sequence in streaming responses as per spec example: https://platform.openai.com/docs/api-reference/responses/create

[Copilot]

Pull Request Overview

Reformats the streaming response event sequence to align with the OpenAI Responses API spec by introducing additional intermediate event types and altering the emitted event order.

• Adds new streaming event model classes (content part, text delta/done, in-progress).
• Adjusts processor logic to emit the new event sequence.
• Removes prior output_item.added emission but still emits output_item.done (likely unintended).

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 8 comments.

File	Description
ResponseStreamEvent.cs	Adds new derived streaming event classes and updates polymorphic type registrations.
AIAgentResponsesProcessor.cs	Modifies streaming emission logic to use new event types and revised sequencing.

[stephentoub]

Why is this necessary?

[stephentoub]

nit:

Suggested change

	Dictionary<string, int> messageIdOutputIndexes = new();
	Dictionary<string, int> messageIdOutputIndexes = [];

[stephentoub]

Why is this using "<null>"? Is that just an arbitrary sentinel to use to group together all messages without IDs?

[ReubenBond]

This is superseded by #1550

[DeagleGross]

was intended to be a quick fix for compatibility with openai responses streaming; but Reuben has already provided a stable change (see #1550); closing