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/thread-manager-sample/src/main.rs

@@ -242,7 +242,6 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         tool_output_token_limit: None,
         agents_enabled: true,
         agent_max_threads: Some(6),
-        agent_support_agent_type: None,
         agent_default_subagent_model: None,
         agent_default_subagent_reasoning_effort: None,
         agent_job_max_runtime_seconds: None,