Reserve the tool_search namespace for the search tool (#37188)

## What changed

- Remove namespace tools named `tool_search` before registering the built-in
  search tool, preventing them from sharing its model-visible surface.
- Record removed namespace tools as collisions so strict collision handling
  reports their qualified tool names.

## Testing

- Cover direct and deferred MCP tools that use the reserved namespace.
- Cover strict collision errors for both direct and deferred conflicts.

GitOrigin-RevId: 28fff918b4358a67b167467d92728fa545b951d7

codex-rs/core/src/tools/spec_plan.rs

@@ -342,7 +342,23 @@ pub(crate) fn finalize_tool_router(
         })
     {
         if registry.remove(&tool_search_name).is_some() {
-            registry.record_collision(tool_search_name);
+            registry.record_collision(tool_search_name.clone());
+        }
+        // Special model tools own the namespace matching their wire identity, so
+        // regular namespace tools cannot advertise that same model-visible surface.
+        let conflicting_tool_names = registry
+            .entries()
+            .filter_map(|tool| {
+                let ToolSpec::Namespace(namespace) = tool.runtime.spec() else {
+                    return None;
+                };
+                (namespace.name == tool_search_name.name).then(|| tool.runtime.tool_name())
+            })
+            .collect::<Vec<_>>();
+        for tool_name in conflicting_tool_names {
+            if registry.remove(&tool_name).is_some() {
+                registry.record_collision(tool_name);
+            }
         }
         append_tool_search_executor(turn_context, &mut registry, tool_search_handler_cache);
     }

codex-rs/core/src/tools/spec_plan_tests.rs

@@ -1161,6 +1161,50 @@ async fn mcp_and_tool_search_follow_direct_and_deferred_tool_exposure() {
         "tool_search",
         &ToolName::namespaced("mcp__searchable", "lookup").to_string(),
     ]);
+
+    let reserved_namespace = probe_with(
+        |turn| {
+            set_feature(turn, Feature::CodeMode, /*enabled*/ true);
+            turn.model_info.supports_search_tool = true;
+        },
+        ToolPlanInputs {
+            tool_runtimes: vec![
+                mcp_runtime(
+                    "reserved_direct",
+                    "tool_search",
+                    "inspect",
+                    ToolExposure::Direct,
+                ),
+                mcp_runtime(
+                    "reserved_deferred",
+                    "tool_search",
+                    "tool_search_tool",
+                    ToolExposure::Deferred,
+                ),
+                mcp_runtime(
+                    "searchable",
+                    "mcp__searchable",
+                    "lookup",
+                    ToolExposure::Deferred,
+                ),
+            ],
+            ..ToolPlanInputs::default()
+        },
+    )
+    .await;
+    reserved_namespace.assert_visible_contains(&["tool_search"]);
+    reserved_namespace.assert_registered_contains(&[
+        "tool_search",
+        &ToolName::namespaced("mcp__searchable", "lookup").to_string(),
+    ]);
+    reserved_namespace.assert_registered_lacks(&[
+        &ToolName::namespaced("tool_search", "inspect").to_string(),
+        &ToolName::namespaced("tool_search", "tool_search_tool").to_string(),
+    ]);
+    assert!(matches!(
+        reserved_namespace.visible_spec("tool_search"),
+        ToolSpec::ToolSearch { .. }
+    ));
 }
 
 #[tokio::test]
@@ -1301,6 +1345,37 @@ async fn strict_tool_collisions_reject_external_and_synthetic_duplicates() {
             false,
             true,
         ),
+        (
+            "tool_search.tool_search_tool",
+            ToolPlanInputs {
+                tool_runtimes: vec![mcp_runtime(
+                    "reserved",
+                    "tool_search",
+                    "tool_search_tool",
+                    ToolExposure::Deferred,
+                )],
+                ..ToolPlanInputs::default()
+            },
+            false,
+            true,
+        ),
+        (
+            "tool_search.inspect",
+            ToolPlanInputs {
+                tool_runtimes: vec![
+                    mcp_runtime("reserved", "tool_search", "inspect", ToolExposure::Direct),
+                    mcp_runtime(
+                        "searchable",
+                        "mcp__searchable",
+                        "lookup",
+                        ToolExposure::Deferred,
+                    ),
+                ],
+                ..ToolPlanInputs::default()
+            },
+            false,
+            true,
+        ),
     ];
 
     let namespace_cases = [