Python: agents: ChatAgent.__init__ chat_client as Callable factory

Consider if chat_client parameter could be a Callable[..., ChatClientProtocol] to allow lazy instantiation.

**File:** python/packages/core/agent_framework/_agents.py
**Location:** ChatAgent class, __init__ method, chat_client parameter (Line ~592)

**Current:** chat_client: ChatClientProtocol
**Question:** Could this be Callable[..., ChatClientProtocol]?

This would allow deferred client creation or factory patterns.

Extracted from #3096
Parent issue: #3575