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/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);