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/tools/src/tool_config.rs

@@ -117,6 +117,7 @@ pub struct ToolsConfig {
     pub collab_tools: bool,
     pub goal_tools: bool,
     pub multi_agent_v2: bool,
+    pub multi_agent_v2_non_code_mode_only: bool,
     pub hide_spawn_agent_metadata: bool,
     pub spawn_agent_usage_hint: bool,
     pub spawn_agent_usage_hint_text: Option<String>,
@@ -257,6 +258,7 @@ impl ToolsConfig {
             collab_tools: include_collab_tools,
             goal_tools: include_goal_tools,
             multi_agent_v2: include_multi_agent_v2,
+            multi_agent_v2_non_code_mode_only: false,
             hide_spawn_agent_metadata: false,
             spawn_agent_usage_hint: true,
             spawn_agent_usage_hint_text: None,
@@ -314,6 +316,15 @@ impl ToolsConfig {
         self
     }
 
+    pub fn with_multi_agent_v2_non_code_mode_only(
+        mut self,
+        multi_agent_v2_non_code_mode_only: bool,
+    ) -> Self {
+        self.multi_agent_v2_non_code_mode_only =
+            self.multi_agent_v2 && multi_agent_v2_non_code_mode_only;
+        self
+    }
+
     pub fn with_goal_tools_allowed(mut self, allowed: bool) -> Self {
         self.goal_tools = self.goal_tools && allowed;
         self