## Why
Model overrides for `spawn_agent` must be compatible with the multi-agent backend used by the current turn.
## What changed
- Carry each model's multi-agent backend metadata into `ModelPreset`.
- Filter the advertised `spawn_agent` model overrides for multi-agent v2 and reject overrides assigned to another backend.
- Limit error suggestions to picker-visible, backend-compatible models.
## Testing
Added coverage for hiding incompatible models from the tool description and rejecting them during spawn validation.
GitOrigin-RevId: 22c12aba67df46e9743a74f019b72a1b8b76b308