ACP: let hosts answer ask_user and exit_plan_mode

[benjaminwestern]

ACP needs a host-answerable input request for ask_user and exit_plan_mode.

Right now ACP can start sessions, resume them, send prompts, cancel turns, and handle permission flows. But if Gemini CLI needs structured user input, ACP does not give the host a way to answer it. That forces the flow back into the terminal UI.

That is the gap I need closed.

I want Gemini CLI in ACP mode to work like the other controller boundaries I already use:

• Codex app-server
• Claude Agent SDK

The host should be able to:

• receive the input request
• show it in its own UI
• send the answer back
• continue the same turn

Relevant ACP work I checked first:

• feat(cli): conditionally exclude ask_user tool in ACP mode #23045
• ACP: no tool_call session update before request_permission #21783
• ACP mode tool confirmations are delayed by ~30s due to MessageBus timeout #17952
• fix(acp): Update allow approval policy flow for ACP clients to fix config persistence and compatible with TUI #23818
• feat(acp): Implemented the structured terminal lifecycle for ACP. #23961
• Gemini CLI in ACP mode prompts for login when launched from Python, but not from terminal #12042

This PR is the narrow fix for that gap: #24664

What it adds:

• an opt-in ACP extension: gemini/requestUserInput
• host-answerable handling for ask_user
• host-answerable handling for exit_plan_mode
• ask_user stays disabled in ACP unless the host explicitly opts in
• per-kind support so a host can enable only exit_plan_mode if it wants

What it does not do:

• add MCP
• add a second app-server
• turn ask_user back on globally in ACP