Python: types: Clarify if author_name applies to ChatResponse or only AgentResponse

[eavanvalkenburg]

Clarify whether the author_name attribute should apply to ChatResponse or only to AgentResponse.

File: python/packages/core/agent_framework/_types.py
Location: ChatResponseUpdate.author_name (around line ~2321)

Action: Document/clarify the intended usage of author_name.

Parent issue: #3575