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

[guibor]

Any plans to merge this?

[gaoyuan1982]

We need this feature. Any plan to merge it?

[pradelgorithm]

need this merged in asap

[priyashpatil]

Hey @mollehxh ,

Would you be able to resolve the merge conflict?

[svenit]

Please merge it!

[benbrandt]

Hi thanks for taking the time to dig in!
Development is moving to agentclientprotocol/codex-acp. The new adapter is built on the new Codex App Server, and we are pooling implementation and maintenance work across teams there. If this is still needed, please open a PR for that one. Thanks!