Add ACP plan mode support

[mollehxh]

Summary

This PR adds Plan mode support in ACP and fixes Plan-mode UX in clients that do not render structured plan items natively.

Changes

• Added collaboration mode listing via ModelsManager.
• Added session config option collaboration_mode.
• Implemented collaboration mode switching through Op::OverrideTurnContext { collaboration_mode: ... }.
• Tracked current collaboration mode from TurnStarted events.
• Added handling for EventMsg::RequestUserInput.
• Added compatibility bridge from request_user_input to session/request_permission and submit of Op::UserInputAnswer.
• Marked fallback user-input tool calls as Completed after final answer (so they do not stay visually pending).
• Added fallback rendering for TurnItem::Plan on ItemCompleted: plan text is emitted as normal agent chat text for ACP clients without native plan item UI.

Fixes #182