[codex] Keep v1 spawn metadata visible (#26599)

## Summary
- keep the legacy v1 `spawn_agent` role and model selectors visible
- add regression coverage for the default v1 tool plan

## Why
`hide_spawn_agent_metadata` is a multi-agent v2 setting, but the v1
planning branch also consumed it. After the default changed to `true`,
v1 stopped advertising `agent_type`, `model`, `reasoning_effort`, and
`service_tier`, preventing configured agents from being selected.

This keeps the hidden-metadata default for v2 while opting v1 out of
that behavior.

Fixes #26363.

## Validation
Not run locally, per request; CI will validate the change.

codex-rs/core/src/tools/spec_plan.rs

@@ -762,10 +762,7 @@ fn add_collaboration_tools(context: &CoreToolPlanContext<'_>, planned_tools: &mu
                 SpawnAgentHandler::new(SpawnAgentToolOptions {
                     available_models: turn_context.available_models.clone(),
                     agent_type_description,
-                    hide_agent_type_model_reasoning: turn_context
-                        .config
-                        .multi_agent_v2
-                        .hide_spawn_agent_metadata,
+                    hide_agent_type_model_reasoning: false,
                     include_usage_hint: turn_context.config.multi_agent_v2.usage_hint_enabled,
                     usage_hint_text: turn_context.config.multi_agent_v2.usage_hint_text.clone(),
                     max_concurrent_threads_per_session: max_concurrent_threads_per_session(

codex-rs/core/src/tools/spec_plan_tests.rs

@@ -1003,6 +1003,30 @@ async fn multi_agent_feature_selects_one_agent_tool_family() {
             "wait_agent".to_string(),
         ]
     );
+    let ToolSpec::Namespace(namespace) = v1.visible_spec(MULTI_AGENT_V1_NAMESPACE) else {
+        panic!("expected v1 multi-agent namespace");
+    };
+    let Some(ResponsesApiNamespaceTool::Function(spawn_agent)) =
+        namespace.tools.iter().find(|tool| {
+            matches!(
+                tool,
+                ResponsesApiNamespaceTool::Function(tool) if tool.name == "spawn_agent"
+            )
+        })
+    else {
+        panic!("expected v1 spawn_agent function");
+    };
+    let properties = spawn_agent
+        .parameters
+        .properties
+        .as_ref()
+        .expect("spawn_agent should use object params");
+    for property in ["agent_type", "model", "reasoning_effort", "service_tier"] {
+        assert!(
+            properties.contains_key(property),
+            "expected v1 spawn_agent to expose `{property}`"
+        );
+    }
 
     let v2 = probe(|turn| {
         set_feature(turn, Feature::MultiAgentV2, /*enabled*/ true);