fix: align AgentSideConnection.to_agent type with AgentFactory

[extreme-structure]

Problem

AgentSideConnection declares its factory parameter as Callable[[Client], Agent] | Agent, but _prepare always invokes it with the AgentSideConnection itself (to_agent(self)). The narrower AgentFactory = Callable[[AgentSideConnection], Agent] used by AcpServer is therefore not assignable to it under parameter contravariance.

Result: a strict type-checker consumer (mypy --strict / ty) that passes an AgentFactory into AgentSideConnection gets a spurious error at the call site — e.g. AcpServer._handle_initialize's AgentSideConnection(self.agent_factory, transport) — and has to insert a cast.

Fix

Narrow the four to_agent declarations to Callable[[AgentSideConnection], Agent] | Agent so they match AgentFactory:

• AgentSideConnection.__init__
• AgentSideConnection.attach
• AgentSideConnection._prepare
• spawn_client_process

Also drop the now-unused Client import from agent/connection.py.

Notes

• The symmetric client side (ClientSideConnection.__init__ / spawn_agent_process) uses to_client: Callable[[Agent], Client] | Client where the factory is actually invoked with the ClientSideConnection itself — a similar but currently self-consistent imprecision, left out of scope here.
• I wasn't able to run ty check in my environment (dependency fetch timed out); the change is annotation-only and py_compile passes.