.Net Agents - Support single-agent / no-chat invocation of `OpenAIAssistantAgent`

[crickman]

Motivation and Context

Support pattern for direct invocation of OpenAIAssistantAgent in alignment with ChatCompletionAgent

Description

• Add instance method OpenAIAssistantAgent.InvokeAsync for a specified thread-id
• Add instance method OpenAIAssistantAgent.AddChatMessageAsync for a specified thread-id
• Add thread management methods to: Create and Delete a thread.
• Refactored thread related actions to static class AssistantThreadActions so it may be utilized by both OpenAIAssistantAgent and OpenAIAssistantChannel
• Updated samples to demonstrate no-chat invocation pattern

Contribution Checklist

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