Restrict spawned-agent models to the active backend (#32751)

## 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

codex-rs/protocol/src/openai_models.rs

@@ -230,6 +230,11 @@ pub struct ModelPreset {
     pub upgrade: Option<ModelUpgrade>,
     /// Whether this preset should appear in the picker UI.
     pub show_in_picker: bool,
+    /// Multi-agent backend selected when this model starts a new thread.
+    #[serde(default, skip_serializing, skip_deserializing)]
+    #[schemars(skip)]
+    #[ts(skip)]
+    pub multi_agent_version: Option<MultiAgentVersion>,
     /// Availability NUX shown when this preset becomes accessible to the user.
     pub availability_nux: Option<ModelAvailabilityNux>,
     /// whether this model is supported in the api
@@ -606,6 +611,7 @@ impl From<ModelInfo> for ModelPreset {
                 migration_markdown: Some(upgrade.migration_markdown.clone()),
             }),
             show_in_picker: info.visibility == ModelVisibility::List,
+            multi_agent_version: info.multi_agent_version,
             availability_nux: info.availability_nux,
             supported_in_api: info.supported_in_api,
             input_modalities: info.input_modalities,