.Net Agents - Fix and streamlining for OpenAIAssistantAgent

[crickman]

Motivation and Context

Adjust design approach for OpenAIAssistantAgent to match that of AzureAIAgent, specifically around exposing the public constructor and not requiring the use of static factory methods. This approach enables all features present in the underlying SDK and addresses several initialization bugs based on the hidden/private constructor.

Obsolete methods are maintained for compatibility, but will not be graduated. Eventually, it is likely they will be removed.

Description

Changes:

• Exposed public constructor for OpenAIAssistantAgent
• Now support all assistant features (available in the SDK)
• Deprecated factory methods and functional wrappers
• Exposed extension methods to facilitate common operations
• Adjusted all samples and tests
• No single sample increased its line count...most have less lines of code

Fixes:

• Fixes: .Net Bug: LoggerFactory is never set in Agent #10110
• Fixes: .Net Bug: OpenAIAssistantAgent LoggerFactory #10160
• Fixes: .Net: Inconsistent API's in the Agent Framework #10390
• Fixes: .Net Agents - Expose public constructor for OpenAIAssistantAgent #10397
• Fixes: .Net Agents Assistants V2 - Support StructuredOutput #8222
• Fixes: .Net Agents: Assistants V2 - Support tool_choice #6795

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 😄