acp: Support specifying settings for extensions (#43177)

This allows you to specify default_model and default_mode for ACP
extensions, e.g.
```
"auggie": {
  "default_model": "gpt-5",
  "default_mode": "default",
  "type": "extension"
},
```

Release Notes:

- Added support for specifying settings for ACP extensions
(`default_mode`, `default_model`)

crates/remote_server/src/remote_editing_tests.rs

@@ -1838,6 +1838,7 @@ async fn test_remote_external_agent_server(
                 &json!({
                     "agent_servers": {
                         "foo": {
+                            "type": "custom",
                             "command": "foo-cli",
                             "args": ["--flag"],
                             "env": {