Make multi-agent v2 tool namespace configurable (#23147)

## Summary
- Add `features.multi_agent_v2.tool_namespace` with config/schema
validation for Responses-compatible namespace values.
- Thread the resolved namespace into `ToolsConfig` for normal turns and
review turns.
- Wrap MultiAgentV2 tool specs and registry names in the configured
namespace when namespace tools are supported, while falling back to the
plain tool names when they are not.

## Validation
- `just fmt`
- `just write-config-schema`
- `cargo test -p codex-features multi_agent_v2_feature_config --
--nocapture`
- `cargo test -p codex-core test_build_specs_multi_agent_v2 --
--nocapture`
- `cargo test -p codex-core multi_agent_v2_config -- --nocapture`
- `cargo test -p codex-core
multi_agent_v2_rejects_invalid_tool_namespace -- --nocapture`
- `cargo test -p codex-tools`
- `git diff --check`

codex-rs/tools/src/tool_config.rs

@@ -121,6 +121,7 @@ pub struct ToolsConfig {
     pub hide_spawn_agent_metadata: bool,
     pub spawn_agent_usage_hint: bool,
     pub spawn_agent_usage_hint_text: Option<String>,
+    pub multi_agent_v2_tool_namespace: Option<String>,
     pub max_concurrent_threads_per_session: Option<usize>,
     pub wait_agent_min_timeout_ms: Option<i64>,
     pub wait_agent_max_timeout_ms: Option<i64>,
@@ -260,6 +261,7 @@ impl ToolsConfig {
             hide_spawn_agent_metadata: false,
             spawn_agent_usage_hint: true,
             spawn_agent_usage_hint_text: None,
+            multi_agent_v2_tool_namespace: None,
             max_concurrent_threads_per_session: None,
             wait_agent_min_timeout_ms: None,
             wait_agent_max_timeout_ms: None,
@@ -316,6 +318,14 @@ impl ToolsConfig {
         self
     }
 
+    pub fn with_multi_agent_v2_tool_namespace(
+        mut self,
+        multi_agent_v2_tool_namespace: Option<String>,
+    ) -> Self {
+        self.multi_agent_v2_tool_namespace = multi_agent_v2_tool_namespace;
+        self
+    }
+
     pub fn with_multi_agent_v2_non_code_mode_only(
         mut self,
         multi_agent_v2_non_code_mode_only: bool,