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/core/src/config/config_tests.rs

@@ -9656,6 +9656,7 @@ 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
 "#,
     )?;
 
@@ -9683,6 +9684,7 @@ hide_spawn_agent_metadata = true
         Some("Subagent guidance.")
     );
     assert!(config.multi_agent_v2.hide_spawn_agent_metadata);
+    assert!(config.multi_agent_v2.non_code_mode_only);
 
     Ok(())
 }
@@ -9702,6 +9704,7 @@ usage_hint_text = "base hint"
 root_agent_usage_hint_text = "base root hint"
 subagent_usage_hint_text = "base subagent hint"
 hide_spawn_agent_metadata = true
+non_code_mode_only = false
 
 [profiles.no_hint.features.multi_agent_v2]
 max_concurrent_threads_per_session = 6
@@ -9711,6 +9714,7 @@ usage_hint_text = "profile hint"
 root_agent_usage_hint_text = "profile root hint"
 subagent_usage_hint_text = "profile subagent hint"
 hide_spawn_agent_metadata = false
+non_code_mode_only = true
 "#,
     )?;
 
@@ -9736,6 +9740,7 @@ hide_spawn_agent_metadata = false
         Some("profile subagent hint")
     );
     assert!(!config.multi_agent_v2.hide_spawn_agent_metadata);
+    assert!(config.multi_agent_v2.non_code_mode_only);
 
     Ok(())
 }
@@ -9759,6 +9764,7 @@ enabled = true
     assert_eq!(config.multi_agent_v2.max_concurrent_threads_per_session, 4);
     assert_eq!(config.multi_agent_v2.min_wait_timeout_ms, 10_000);
     assert_eq!(config.agent_max_threads, Some(3));
+    assert!(!config.multi_agent_v2.non_code_mode_only);
 
     Ok(())
 }

codex-rs/core/src/config/mod.rs

@@ -846,6 +846,7 @@ pub struct MultiAgentV2Config {
     pub root_agent_usage_hint_text: Option<String>,
     pub subagent_usage_hint_text: Option<String>,
     pub hide_spawn_agent_metadata: bool,
+    pub non_code_mode_only: bool,
 }
 
 impl Default for MultiAgentV2Config {
@@ -859,6 +860,7 @@ impl Default for MultiAgentV2Config {
             root_agent_usage_hint_text: None,
             subagent_usage_hint_text: None,
             hide_spawn_agent_metadata: false,
+            non_code_mode_only: false,
         }
     }
 }
@@ -1995,6 +1997,10 @@ fn resolve_multi_agent_v2_config(
         .and_then(|config| config.hide_spawn_agent_metadata)
         .or_else(|| base.and_then(|config| config.hide_spawn_agent_metadata))
         .unwrap_or(default.hide_spawn_agent_metadata);
+    let non_code_mode_only = profile
+        .and_then(|config| config.non_code_mode_only)
+        .or_else(|| base.and_then(|config| config.non_code_mode_only))
+        .unwrap_or(default.non_code_mode_only);
 
     MultiAgentV2Config {
         max_concurrent_threads_per_session,
@@ -2004,6 +2010,7 @@ fn resolve_multi_agent_v2_config(
         root_agent_usage_hint_text,
         subagent_usage_hint_text,
         hide_spawn_agent_metadata,
+        non_code_mode_only,
     }
 }