Namespace multi-agent v2 tools under collaboration (#29067)

## Summary

Multi-agent v2 tools now use the fixed `collaboration` namespace when
namespace tools are available. This keeps the model-visible hint and the
actual tool surface aligned around `functions.collaboration.*`, without
exposing an unshipped namespace knob to users.

The PR also removes the old `features.multi_agent_v2.tool_namespace`
config/schema surface, updates the MAv2 test fixtures for namespaced
calls, and fixes stale `TurnContext.features` references that were
breaking `codex-core` builds.

## Changes

- Expose MAv2 tools under `collaboration` instead of relying on a
configurable namespace.
- Remove `tool_namespace` from MAv2 TOML config, resolved config,
validation, schema, and tests.
- Update tool-planning and integration fixtures to assert or emit
namespaced MAv2 tool calls.
- Read feature state through `TurnContext.config.features` in the
multi-agent mode context paths.

## Testing

- `just write-config-schema`
- `just test -p codex-features`

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

@@ -100,6 +100,7 @@ const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs
 const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
 const TEST_ORIGINATOR: &str = "codex_vscode";
 const LOCAL_PRAGMATIC_TEMPLATE: &str = "You are a deeply pragmatic, effective software engineer.";
+const MULTI_AGENT_V2_NAMESPACE: &str = "collaboration";
 const INVALID_REQUEST_ERROR_CODE: i64 = -32600;
 const TINY_PNG_BYTES: &[u8] = &[
     137, 80, 78, 71, 13, 10, 26, 10, 0, 0, 0, 13, 73, 72, 68, 82, 0, 0, 0, 1, 0, 0, 0, 1, 8, 6, 0,
@@ -3650,7 +3651,12 @@ async fn direct_input_to_multi_agent_v2_subagent_is_rejected() -> Result<()> {
         |req: &wiremock::Request| body_contains(req, PARENT_PROMPT),
         responses::sse(vec![
             responses::ev_response_created("resp-parent-1"),
-            responses::ev_function_call(SPAWN_CALL_ID, "spawn_agent", &spawn_args),
+            responses::ev_function_call_with_namespace(
+                SPAWN_CALL_ID,
+                MULTI_AGENT_V2_NAMESPACE,
+                "spawn_agent",
+                &spawn_args,
+            ),
             responses::ev_completed("resp-parent-1"),
         ]),
     )