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());