Revise input request part to live solely in turn state

[connor4312]

Followup to #325 for 0.6.0. Initially when AHP was authored, MCP servers allowed elicitation at any time. Since the 2026-06 spec revision, they are only allowed in the context of tool calls, which generally means in the context of turns, modulo MCP App-triggered tools. I think we should simplify our model by:

• Removing the chat-level input request list and normalizing to the turn-level input requests
• Move from the transcript-style response part that was added to an active part which:
  • Has response be optional for the still-pending state
  • Has some isSubmitted: boolean to indicate whether the response was submitted (false = being in a draft state)