Support leaf models in multi-agent v2 (#36892)

## What changed

- Allow multi-agent v2 parents to spawn any visible model that has not explicitly disabled multi-agent support.
- Expose collaboration tools to child agents only when their selected model supports multi-agent v2, keeping legacy models as leaf workers.
- Preserve a worker's selected model when reloading it into residency.
- Propagate multi-agent capability metadata to Amazon Bedrock model entries so delegation is gated consistently.

## Testing

- Cover model selection, leaf-worker tool visibility, Bedrock capability handling, and model preservation after reload.

GitOrigin-RevId: 1457adb1a09806d1f0621311d5a42a6815b9dd4e

codex-rs/core/src/agent/control_tests.rs

@@ -649,13 +649,16 @@ async fn ensure_v2_agent_loaded_reloads_registered_unloaded_agent() {
     let (home, mut config) = test_config().await;
     let _ = config.features.enable(Feature::MultiAgentV2);
     let _ = config.features.enable(Feature::Sqlite);
+    config.model = Some("gpt-5.6-sol".to_string());
     let harness = AgentControlHarness::new_with_config(home, config).await;
     let (parent_thread_id, _parent_thread) = harness.start_paginated_thread().await;
     let agent_path = AgentPath::try_from("/root/worker").expect("agent path");
+    let mut child_config = harness.config.clone();
+    child_config.model = Some("gpt-5.6-luna".to_string());
     let spawned_agent = harness
         .control
         .spawn_agent_with_metadata(
-            harness.config.clone(),
+            child_config,
             text_input("hello child"),
             Some(SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
                 parent_thread_id,
@@ -715,11 +718,16 @@ async fn ensure_v2_agent_loaded_reloads_registered_unloaded_agent() {
         .ensure_v2_agent_loaded(harness.config.clone(), spawned_agent.thread_id)
         .await
         .expect("known v2 agent should reload");
-    let _ = harness
+    let reloaded_child = harness
         .manager
         .get_thread(spawned_agent.thread_id)
         .await
         .expect("reloaded child thread should exist");
+    assert_eq!(
+        reloaded_child.config_snapshot().await.model,
+        "gpt-5.6-luna",
+        "residency reload must preserve the worker model instead of inheriting its parent model",
+    );
 
     let communication = InterAgentCommunication::new(
         AgentPath::root(),