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/core/src/config/mod.rs

@@ -246,7 +246,8 @@ Payload:
 ```
 You may also see them addressed as to=/root/..., which indicates your identity is /root/...
 "#;
-const DEFAULT_MULTI_AGENT_V2_SHARED_USAGE_HINT_TEXT: &str = r#"Note that collaboration tools cannot be called from inside `functions.exec`. Call `spawn_agent`, `send_message`, `followup_task`, `wait_agent`, `interrupt_agent`, and `list_agents` only as direct tool calls using the recipient shown in their tool definitions, such as `to=functions.spawn_agent` without a configured namespace or `to=functions.agents.spawn_agent` with `tool_namespace = "agents"`, since they are intentionally absent from the `functions.exec` `tools.*` namespace. Available tools in `functions.exec` are explicitly described with a `tools` namespace in the developer message.
+const DEFAULT_MULTI_AGENT_V2_TOOL_NAMESPACE: &str = "collaboration";
+const DEFAULT_MULTI_AGENT_V2_SHARED_USAGE_HINT_TEXT: &str = r#"Note that collaboration tools cannot be called from inside `functions.exec`. Call `spawn_agent`, `send_message`, `followup_task`, `wait_agent`, `interrupt_agent`, and `list_agents` only as direct tool calls using the recipient shown in their tool definitions, such as `to=functions.collaboration.spawn_agent`, since they are intentionally absent from the `functions.exec` `tools.*` namespace. Available tools in `functions.exec` are explicitly described with a `tools` namespace in the developer message.
 
 All agents share the same directory. In detail:
 - All agents have access to the same container and filesystem as you.
@@ -1157,7 +1158,7 @@ impl MultiAgentV2Config {
                 DEFAULT_MULTI_AGENT_V2_SUBAGENT_USAGE_HINT_TEXT,
                 max_concurrent_threads_per_session,
             )),
-            tool_namespace: None,
+            tool_namespace: Some(DEFAULT_MULTI_AGENT_V2_TOOL_NAMESPACE.to_string()),
             hide_spawn_agent_metadata: true,
             non_code_mode_only: true,
         }