Python: Investigate not implementing `AgentProtocol` for `DurableAIAgent`

[larohra]

We currently implement AgentProtocol for DurableAIAgent that the customer accesses. But this TaskT violates AgentProtocol contract. Any reason to use TaskT here instead of AgentResponse?

Originally posted by @dmytrostruk in #3420 (comment)

Opening this issue to track if we need to modify the contract to reduce confusion for the customer.