Gate multi-agent v2 tools independently of collab (#20246)

## Why

`multi_agents_v2` is meant to be independently gated from the older
`collab` feature. The tool registry still treated the
collaboration-style agent tools as `collab`-only, so enabling
`multi_agents_v2` without `collab` omitted the v2 agent tools. Review
and guardian sub-sessions also need to keep agent spawning disabled even
when the outer session has `multi_agents_v2` enabled.

## What changed

- Include the collab-backed agent tools when either `multi_agents_v2` or
`collab` is enabled.
- Explicitly disable `multi_agents_v2` for review and guardian review
sub-sessions, matching the existing `spawn_csv` and `collab`
restrictions.
- Add a registry test that enables `multi_agents_v2`, disables `collab`,
and verifies the v2 agent tools are present while legacy `send_input`
and `resume_agent` remain hidden.

## Testing

- Added
`test_build_specs_multi_agent_v2_does_not_require_collab_feature`.

codex-rs/core/src/guardian/review_session.rs

@@ -931,6 +931,7 @@ pub(crate) fn build_guardian_review_session_config(
     for feature in [
         Feature::SpawnCsv,
         Feature::Collab,
+        Feature::MultiAgentV2,
         Feature::CodexHooks,
         Feature::Apps,
         Feature::Plugins,

codex-rs/core/src/tasks/review.rs

@@ -110,6 +110,7 @@ async fn start_review_conversation(
     }
     let _ = sub_agent_config.features.disable(Feature::SpawnCsv);
     let _ = sub_agent_config.features.disable(Feature::Collab);
+    let _ = sub_agent_config.features.disable(Feature::MultiAgentV2);
 
     // Set explicit review rubric for the sub-agent
     sub_agent_config.base_instructions = Some(crate::REVIEW_PROMPT.to_string());

codex-rs/tools/src/tool_config.rs

@@ -143,9 +143,9 @@ impl ToolsConfig {
         let include_apply_patch_tool = features.enabled(Feature::ApplyPatchFreeform);
         let include_code_mode = features.enabled(Feature::CodeMode);
         let include_code_mode_only = include_code_mode && features.enabled(Feature::CodeModeOnly);
-        let include_collab_tools = features.enabled(Feature::Collab);
         let include_goal_tools = features.enabled(Feature::Goals);
         let include_multi_agent_v2 = features.enabled(Feature::MultiAgentV2);
+        let include_collab_tools = include_multi_agent_v2 || features.enabled(Feature::Collab);
         let include_agent_jobs = features.enabled(Feature::SpawnCsv);
         let include_search_tool =
             model_info.supports_search_tool && features.enabled(Feature::ToolSearch);

codex-rs/tools/src/tool_registry_plan_tests.rs

@@ -385,6 +385,46 @@ fn test_build_specs_multi_agent_v2_uses_task_names_and_hides_resume() {
     assert_lacks_tool_name(&tools, "resume_agent");
 }
 
+#[test]
+fn test_build_specs_multi_agent_v2_does_not_require_collab_feature() {
+    let model_info = model_info();
+    let mut features = Features::with_defaults();
+    features.disable(Feature::Collab);
+    features.enable(Feature::MultiAgentV2);
+    assert!(!features.enabled(Feature::Collab));
+    let available_models = Vec::new();
+    let tools_config = ToolsConfig::new(&ToolsConfigParams {
+        model_info: &model_info,
+        available_models: &available_models,
+        features: &features,
+        image_generation_tool_auth_allowed: true,
+        web_search_mode: Some(WebSearchMode::Cached),
+        session_source: SessionSource::Cli,
+        permission_profile: &PermissionProfile::Disabled,
+        windows_sandbox_level: WindowsSandboxLevel::Disabled,
+    });
+    let (tools, _) = build_specs(
+        &tools_config,
+        /*mcp_tools*/ None,
+        /*deferred_mcp_tools*/ None,
+        &[],
+    );
+
+    assert_contains_tool_names(
+        &tools,
+        &[
+            "spawn_agent",
+            "send_message",
+            "followup_task",
+            "wait_agent",
+            "close_agent",
+            "list_agents",
+        ],
+    );
+    assert_lacks_tool_name(&tools, "send_input");
+    assert_lacks_tool_name(&tools, "resume_agent");
+}
+
 #[test]
 fn test_build_specs_enable_fanout_enables_agent_jobs_and_collab_tools() {
     let model_info = model_info();