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/core/src/config/mod.rs

@@ -953,6 +953,7 @@ pub struct MultiAgentV2Config {
     pub usage_hint_text: Option<String>,
     pub root_agent_usage_hint_text: Option<String>,
     pub subagent_usage_hint_text: Option<String>,
+    pub tool_namespace: Option<String>,
     pub hide_spawn_agent_metadata: bool,
     pub non_code_mode_only: bool,
 }
@@ -969,6 +970,7 @@ impl Default for MultiAgentV2Config {
             usage_hint_text: None,
             root_agent_usage_hint_text: None,
             subagent_usage_hint_text: None,
+            tool_namespace: None,
             hide_spawn_agent_metadata: false,
             non_code_mode_only: false,
         }
@@ -2178,6 +2180,11 @@ fn resolve_multi_agent_v2_config(
         .or_else(|| base.and_then(|config| config.subagent_usage_hint_text.as_ref()))
         .cloned()
         .or(default.subagent_usage_hint_text);
+    let tool_namespace = profile
+        .and_then(|config| config.tool_namespace.as_ref())
+        .or_else(|| base.and_then(|config| config.tool_namespace.as_ref()))
+        .cloned()
+        .or(default.tool_namespace);
     let hide_spawn_agent_metadata = profile
         .and_then(|config| config.hide_spawn_agent_metadata)
         .or_else(|| base.and_then(|config| config.hide_spawn_agent_metadata))
@@ -2196,6 +2203,7 @@ fn resolve_multi_agent_v2_config(
         usage_hint_text,
         root_agent_usage_hint_text,
         subagent_usage_hint_text,
+        tool_namespace,
         hide_spawn_agent_metadata,
         non_code_mode_only,
     }
@@ -2290,6 +2298,69 @@ fn validate_multi_agent_v2_wait_timeout(label: &str, value: i64) -> std::io::Res
     Ok(())
 }
 
+fn validate_multi_agent_v2_tool_namespace(namespace: Option<&str>) -> std::io::Result<()> {
+    const LABEL: &str = "features.multi_agent_v2.tool_namespace";
+    const MAX_LEN: usize = 64;
+    const RESERVED_RESPONSES_NAMESPACES: &[&str] = &[
+        "api_tool",
+        "browser",
+        "computer",
+        "container",
+        "file_search",
+        "functions",
+        "image_gen",
+        "multi_tool_use",
+        "python",
+        "python_user_visible",
+        "submodel_delegator",
+        "terminal",
+        "tool_search",
+        "web",
+    ];
+
+    let Some(namespace) = namespace else {
+        return Ok(());
+    };
+    if namespace.is_empty() {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            format!("{LABEL} must not be empty"),
+        ));
+    }
+    if namespace.trim() != namespace {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            format!("{LABEL} must not have leading or trailing whitespace"),
+        ));
+    }
+    if !namespace
+        .bytes()
+        .all(|byte| byte.is_ascii_alphanumeric() || matches!(byte, b'_' | b'-'))
+    {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            format!("{LABEL} must match ^[a-zA-Z0-9_-]+$"),
+        ));
+    }
+    if namespace.chars().count() > MAX_LEN {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            format!("{LABEL} must be at most {MAX_LEN} characters"),
+        ));
+    }
+    if namespace == "mcp"
+        || namespace.starts_with("mcp__")
+        || RESERVED_RESPONSES_NAMESPACES.contains(&namespace)
+    {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            format!("{LABEL} uses a reserved namespace: {namespace}"),
+        ));
+    }
+
+    Ok(())
+}
+
 impl Config {
     #[cfg(test)]
     async fn load_from_base_config_with_overrides(
@@ -2911,6 +2982,7 @@ impl Config {
                 "features.multi_agent_v2.default_wait_timeout_ms must be at most features.multi_agent_v2.max_wait_timeout_ms",
             ));
         }
+        validate_multi_agent_v2_tool_namespace(multi_agent_v2.tool_namespace.as_deref())?;
         let agent_max_threads_from_config = cfg.agents.as_ref().and_then(|agents| agents.max_threads);
         let agent_max_threads = if features.enabled(Feature::MultiAgentV2) {
             if agent_max_threads_from_config.is_some() {