feat(acp): add session/set_mode handler

[codefromthecrypt]

Summary

The ACP server advertises available modes in session/new and session/load responses but has no session/set_mode handler. Zed's mode selector populates from these responses but mode changes are silently ignored. There are no tests for mode validation or error paths, and the server and provider test fixtures diverge: the server fixture uses raw ACP session IDs while the provider fixture uses goose session IDs, preventing shared test coverage.

This adds mode validation and aligns the test fixtures. session/set_mode validates the mode string and session existence, and rejects mode changes since per-session persistence is not yet implemented (#7603). The ExpectedSessionId trait lets server and provider tests share the same test functions. Both paths exercise identical error cases.

Type of Change

• Bug fix
• Tests

AI Assistance

• This PR was created or reviewed with AI assistance

Testing

Zed (ACP)

Kill goose and clear session/log state:

$ pkill -f goose
$ rm -rf ~/.local/state/goose/logs ~/.local/share/goose/sessions
$ rm -rf ~/Library/Logs/Zed/ ~/Library/Application\ Support/Zed/threads/

Build the release binary:

$ just release-binary

Add to ~/.config/zed/settings.json:

"agent_servers": {
  "goose": {
    "type": "custom",
    "command": "/path/to/goose-2/target/release/goose",
    "args": ["acp", "--with-builtin", "developer"],
    "env": {
      "RUST_LOG": "debug"
    }
  }
}

Mode selection (expected rejection):

Open Zed's agent panel. The mode selector (bottom bar, next to model selector) shows "auto" as the default.

1. Click the mode selector -- all four modes appear (auto, approve, smart_approve, chat) with descriptions
2. Switch to "approve"
3. Verify the rejection in goose logs:

$ grep -h "Mode change not supported" ~/.local/state/goose/logs/cli/*/*.log

{"timestamp":"...","level":"WARN","fields":{"message":"Sending error response","id":"Number(3)","error":"Error { code: -32602: Invalid params, message: \"Invalid params\", data: Some(String(\"Mode change not supported: session is auto, requested approve\")) }"},"target":"sacp::jsonrpc::outgoing_actor",...}

Related Issues

Prerequisite for #7603

Screenshots

Zed supports mode selection, so I used that.

Before

Goose has no mode selection, just model:

After

The selector briefly shows "approve" then reverts to "auto" -- Zed optimistically updates the UI, then silently reverts when the server returns an error. The error exists until modes are session scoped in #7603

[codefromthecrypt]

this PR is too big, so I will split it into two.

1. Persist goose mode to memory and DB on change (mode visibility fix for everything)
2. ACP support of session mode in general (this code has no idea about the DB and leaves only one patch for sync'ing)

[codefromthecrypt]

fyi I rescoped this PR to the ACP prep part of mode support. I have a local branch ready to add session-scope mode (and persist) that will fix #7603

This is a pre-req as doing both in the same change was too blurry, especially as a lot of the session persistence logic was plumbing to server and the Goose UI (not affecting ACP in really any way).

[codefromthecrypt]

so this is a callback which agentic providers can implement to sync their mode for a given session. It is basically the same way permissions integration in ACP works.

[codefromthecrypt]

this is mechanically moving around because things were getting hairy in the older layout, especially as we add things like mode and later other things.

[codefromthecrypt]

will rebase after #7843 which will drift it

[codefromthecrypt]

side note: I notice the whole permissions integration is hairy, but there's a lot of code moving around with high frequency. I'd like to let that settle before refactoring, I think with some message types we can avoid some of the duplication mode/permission logic that is handled both in ACP internally and also somewhat similarly in the agent loops.

[codefromthecrypt]

#7854 is the mode mutablility/persistence thing split off from this. it keeps things less coupled.. when both of these are in, connecting acp to updatable mode will be less to review