ACP: expose the model list as a category:model config option (session/new returns only mode and allow_all) #4880

Describe the feature or problem you'd like to solve

Over copilot --acp, a client has no way to list the models available to the signed-in account or to pick one for a session.

Measured on GitHub Copilot CLI 1.0.85 (macOS arm64):

• session/new returns sessionId, modes and configOptions only. configOptions holds exactly two select options, mode and allow_all. There is no option with category: "model".
• session/load returns the same set.
• Sending /model as a prompt answers: "The model-picker dialog is only available in the interactive CLI."
• --model <id> accepts any string. copilot --acp --model=not_a_real_model_xyz starts, creates a session and completes a prompt with no error, no stderr, and nothing on the wire reports which model actually ran.

So an ACP client (an editor or a desktop app embedding Copilot) can only offer a single "Copilot default" row. It cannot show the models the user's plan allows, and if it hard-codes ids it cannot tell when one silently fell back.

Proposed solution

Advertise the model list the way the ACP spec already provides for it:

1. Include a SessionConfigOption with type: "select" and category: "model" in session/new and session/load responses, listing the ids the account can use and currentValue set to the active model. Grouping by vendor via SessionConfigSelectGroup is fine.
2. Accept session/set_config_option for that option and emit config_option_update on change, so a client can switch models mid-session without respawning.
3. Ideally, reject an unknown --model value (or at least log it to stderr) instead of silently running the default.

This is the same shape the interactive /model picker already has; it just needs to be exposed on the ACP surface.

Example prompts or workflows

An ACP client starts copilot --acp, calls session/new, reads the model config option, shows its values in a picker, and calls session/set_config_option with the user's choice. Today step 3 yields nothing to show.

Additional context

Minimal reproduction (Node 24):

import { spawn } from 'node:child_process'
const c = spawn('copilot', ['--acp', '--no-auto-update'], { stdio: ['pipe', 'pipe', 'inherit'] })
const send = (o) => c.stdin.write(JSON.stringify(o) + '\n')
let buf = ''
c.stdout.on('data', (d) => {
  buf += d
  for (const line of buf.split('\n').slice(0, -1)) {
    const m = JSON.parse(line)
    if (m.id === 1) send({ jsonrpc: '2.0', id: 2, method: 'session/new', params: { cwd: process.cwd(), mcpServers: [] } })
    if (m.id === 2) { console.log(Object.keys(m.result), m.result.configOptions.map((o) => o.id)); c.kill() }
  }
  buf = buf.slice(buf.lastIndexOf('\n') + 1)
})
send({ jsonrpc: '2.0', id: 1, method: 'initialize', params: { protocolVersion: 1, clientCapabilities: { fs: { readTextFile: false, writeTextFile: false } }, clientInfo: { name: 'probe', version: '0' } } })

Prints [ 'sessionId', 'modes', 'configOptions' ] [ 'mode', 'allow_all' ].

Version: GitHub Copilot CLI 1.0.85. ACP SDK used by the client: @agentclientprotocol/sdk 1.4.0.