Add Responses WebSocket support for Codex continuations

[franciscojavierarceo]

Parent tracker: #69
Design context: #65

Summary

Add a Responses WebSocket adapter in agentic-api so Codex-style continuations can use previous_response_id plus marginal input and reach the same replay path as HTTP continuations.

Scope

• Accept Codex-style response.create frames.
• Map previous_response_id into the response-store continuation path.
• Normalize HTTP and WebSocket requests into the same internal Responses request model.
• Emit Responses stream events back over the socket.
• Preserve Codex fallback semantics when a request is not a valid continuation.

Acceptance criteria

• First-turn and continuation WebSocket flows are covered by tests.
• WebSocket continuation reaches the same replay planner as HTTP continuation.
• Invalid continuation requests fall back or fail in a documented way.
• vLLM itself does not need to expose a Responses WebSocket endpoint for the first implementation.