When register_task_env_overrides is called with only a 'cwd' key
(ACP adapter workspace tracking), the task_id should collapse to
'default' so all interactive surfaces (TUI, gateway, dashboard)
share one long-lived container.
Previously, any override registration — even CWD-only — caused
_resolve_container_task_id to return the session key unchanged,
spinning up a separate container per session. This made it
impossible to authenticate into external services once and have
that auth available across all surfaces.
Now only overrides containing isolation keys (docker_image,
modal_image, singularity_image, daytona_image, env_type) trigger
per-task container isolation.
Fixes#37361
