feat: expose multi-agent v2 as model-only tools (#22514)

## Why

`code_mode_only` filters code-mode nested tools out of the top-level
tool list. For multi-agent v2, we need a rollout shape where the
collaboration tools remain callable as normal model tools without also
being embedded into the code-mode `exec` tool declaration.

Related to this:
https://openai-corpws.slack.com/archives/C0AQLHB4U75/p1778660267922549

## What Changed

- Adds `features.multi_agent_v2.non_code_mode_only`, including config
resolution, profile override handling, and generated schema coverage.
- Introduces `ToolExposure::DirectModelOnly` so a tool can be included
in the initial model-visible list while staying out of the nested
code-mode tool surface.
- Applies that exposure to the multi-agent v2 tools when the new flag is
set: `spawn_agent`, `send_message`, `followup_task`, `wait_agent`,
`close_agent`, and `list_agents`.
- Updates code-mode-only filtering so direct-model-only tools remain
visible while ordinary nested code-mode tools are still hidden.

## Verification

- Added config parsing/profile tests for `non_code_mode_only`.
- Added tool spec coverage for the code-mode-only multi-agent v2
exposure behavior.

codex-rs/features/src/feature_configs.rs

@@ -25,6 +25,8 @@ pub struct MultiAgentV2ConfigToml {
     pub subagent_usage_hint_text: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub hide_spawn_agent_metadata: Option<bool>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub non_code_mode_only: Option<bool>,
 }
 
 impl FeatureConfig for MultiAgentV2ConfigToml {

codex-rs/features/src/tests.rs

@@ -481,6 +481,7 @@ usage_hint_text = "Custom delegation guidance."
 root_agent_usage_hint_text = "Root guidance."
 subagent_usage_hint_text = "Subagent guidance."
 hide_spawn_agent_metadata = true
+non_code_mode_only = true
 "#,
     )
     .expect("features table should deserialize");
@@ -500,6 +501,7 @@ hide_spawn_agent_metadata = true
             root_agent_usage_hint_text: Some("Root guidance.".to_string()),
             subagent_usage_hint_text: Some("Subagent guidance.".to_string()),
             hide_spawn_agent_metadata: Some(true),
+            non_code_mode_only: Some(true),
         }))
     );
 }
@@ -535,6 +537,7 @@ usage_hint_enabled = false
             root_agent_usage_hint_text: None,
             subagent_usage_hint_text: None,
             hide_spawn_agent_metadata: None,
+            non_code_mode_only: None,
         }))
     );
 }