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

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 https://github.com/microsoft/agent-framework/pull/3420#discussion_r2728849807_

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

Start by reviewing the AgentProtocol contract and DurableAIAgent implementation, then read the linked discussion in PR #3420. Determine whether TaskT or AgentResponse matches the customer-facing contract, and consider whether the contract itself needs clarification. Done means the contract and implementation are aligned without confusing customers.

Written by the indexing model from the issue text.