Accept DraftSession in AgentSession and Fix client types

[chiragjn]

Note

Low Risk
Typing and import-structure changes only; runtime behavior should be unchanged aside from stricter static types for agent_name on draft sessions.

Overview
Tightens typing across the high-level agents wrappers by replacing typing.Any clients with TrueFoundryGateway / AsyncTrueFoundryGateway, and refactors imports using from __future__ import annotations plus TYPE_CHECKING blocks to cut circular-import pressure (including removing lazy client imports inside Turn).

AgentSession / AsyncAgentSession now accept Session | DraftSession raw payloads, and agent_name is typed as Optional[str] to match draft sessions where the name may be unset.

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