Expose spawn agent types only when roles are configured (#33572)

## Why

The `agent_type` argument is only useful when agent roles are configured. Avoid
advertising an inapplicable override in the model-visible `spawn_agent` schema.

## What changed

- Include `agent_type` in the V1 and V2 spawn tools only when configured roles
  are available.
- Clarify that `agent_type` is an explicit override and document its interaction
  with context forking.
- Remove the unused `agents.support_agent_type` configuration field.

## Testing

- Add coverage for hiding `agent_type` without roles and exposing it when a role
  is configured.

GitOrigin-RevId: 32e0f123b15f317e9cdb71774029375bc073d9ad

codex-rs/core/src/agent/role.rs

@@ -241,10 +241,7 @@ pub(crate) mod spawn_tool_spec {
             }
         }
 
-        format!(
-            "Optional type name for the new agent. If omitted, `{DEFAULT_ROLE_NAME}` is used.\nAvailable roles:\n{}",
-            formatted_roles.join("\n"),
-        )
+        format!("Available roles:\n{}", formatted_roles.join("\n"))
     }
 
     fn format_role(name: &str, declaration: &AgentRoleConfig) -> String {