HttpAgentServerAdapter

Public v1 name for TransportAdapter plus optional high-level capabilities. Renamed to reflect that this interface now denotes the full agent-server protocol contract (not merely wire transport): any object that satisfies it can back a useStream call. See plan-custom-transport.md §4 for the rollout.

The extra optional methods let adapters surface thread state and history without the framework needing to issue a parallel HTTP request — useStream.hydrate() calls getState?() when present and falls back to client.threads.getState otherwise. Adapters that don't know how to produce these values can simply omit them.

The legacy TransportAdapter export is retained for back-compat and resolves to the same structural type; new code should prefer AgentServerAdapter.