Python: ADR for create/get agent API

[dmytrostruk]

Motivation and Context

There is a misalignment between the create/get agent API in the .NET and Python implementations. This PR contains ADR to compare APIs in both languages to decide what needs to be added.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR introduces an ADR (Architecture Decision Record) that documents the misalignment between .NET and Python implementations of the create/get agent API, proposing a solution to achieve API parity.

• Documents current API differences between .NET and Python agent creation/retrieval methods
• Proposes adding missing get_agent and enhanced create_agent methods to Python implementations
• Provides comprehensive comparison table of current .NET provider implementations

[eavanvalkenburg]

I can get behind the principle, my only concern is that the overridden create agent methods will need to be async to work and that's not great

[markwallace-microsoft]

How will this work if I want to create different types of persistent agent at the same time? We would need unique AgentFractory names, right?

[markwallace-microsoft]

Do you have a recommendation?

This would be a good topic to discuss with Brett and Johan. Could you book a meeting with them as part 2 of the API review and review this decision.