Python: [Feature]: Python: add finish reason to AgentResponse and AgentResponseUpdate

[eavanvalkenburg]

Description

Follow the dotnet update in python

Code Sample

Language/SDK

Python

[LEDazzio01]

I'd like to pick up the Python side of this.

Looking at the code, ChatResponse and ChatResponseUpdate already have finish_reason, but AgentResponse and AgentResponseUpdate do not. The plan:

1. Add finish_reason: FinishReasonLiteral | FinishReason | None to both AgentResponse.__init__ and AgentResponseUpdate.__init__
2. Update _process_update() to propagate finish_reason for AgentResponse/AgentResponseUpdate (mirroring the existing ChatResponse logic)
3. Update map_chat_to_agent_update() to forward finish_reason from ChatResponseUpdate
4. Add corresponding unit tests

PR incoming.