.Net: [Agents] Abstraction for streaming

[SergeyMenshykh]

Motivation, Context and Description

Today, the agent abstraction lacks streaming capabilities that would enable agents to stream content provided by LLM back to the calling code. This PR introduces the first step in this direction by reusing chat completion API streaming building blocks, such as StreamingChatMessageContent, StreamingKernelContent, and so on. Later, when the OpenAI Assistants API supports streaming, the abstraction will need to be revised to support streaming for both chat completion and assistant APIs.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄