chore: namespace v1 sub-agent tools (#23475)

## Why

The v1 sub-agent tools are a single tool family, but they were exposed
as separate flat function tools. This makes the model-visible surface
less clearly grouped and leaves the legacy names in the same flat
namespace as newer agent tooling.

## What

- Wraps the v1 `spawn_agent`, `send_input`, `resume_agent`,
`wait_agent`, and `close_agent` specs in the `multi_agent_v1` namespace.
- Registers the corresponding handlers with namespaced runtime tool
names.
- Updates tool-planning, deferred tool search, and sub-agent
notification tests to assert the namespace shape and child `spawn_agent`
lookup.

## Verification

- Updated `codex-core` coverage for the v1 multi-agent tool plan,
deferred tool search output, and sub-agent tool descriptions.

codex-rs/app-server/tests/suite/v2/turn_start.rs

@@ -2910,7 +2910,12 @@ async fn turn_start_emits_spawn_agent_item_with_model_metadata_v2() -> Result<()
         |req: &wiremock::Request| body_contains(req, PARENT_PROMPT),
         responses::sse(vec![
             responses::ev_response_created("resp-turn1-1"),
-            responses::ev_function_call(SPAWN_CALL_ID, "spawn_agent", &spawn_args),
+            responses::ev_function_call_with_namespace(
+                SPAWN_CALL_ID,
+                "multi_agent_v1",
+                "spawn_agent",
+                &spawn_args,
+            ),
             responses::ev_completed("resp-turn1-1"),
         ]),
     )
@@ -3107,7 +3112,12 @@ async fn turn_start_emits_spawn_agent_item_with_effective_role_model_metadata_v2
         |req: &wiremock::Request| body_contains(req, PARENT_PROMPT),
         responses::sse(vec![
             responses::ev_response_created("resp-turn1-1"),
-            responses::ev_function_call(SPAWN_CALL_ID, "spawn_agent", &spawn_args),
+            responses::ev_function_call_with_namespace(
+                SPAWN_CALL_ID,
+                "multi_agent_v1",
+                "spawn_agent",
+                &spawn_args,
+            ),
             responses::ev_completed("resp-turn1-1"),
         ]),
     )