[tool search] support namespaced deferred dynamic tools (#18413)

Deferred dynamic tools need to round-trip a namespace so a tool returned
by `tool_search` can be called through the same registry key that core
uses for dispatch.

This change adds namespace support for dynamic tool specs/calls,
persists it through app-server thread state, and routes dynamic tool
calls by full `ToolName` while still sending the app the leaf tool name.
Deferred dynamic tools must provide a namespace; non-deferred dynamic
tools may remain top-level.

It also introduces `LoadableToolSpec` as the shared
function-or-namespace Responses shape used by both `tool_search` output
and dynamic tool registration, so dynamic tools use the same wrapping
logic in both paths.

Validation:
- `cargo test -p codex-tools`
- `cargo test -p codex-core tool_search`

---------

Co-authored-by: Sayan Sisodiya <sayan@openai.com>

codex-rs/core/tests/suite/search_tool.rs

@@ -730,6 +730,7 @@ async fn tool_search_returns_deferred_dynamic_tool_and_routes_follow_up_call() -
                     "item": {
                         "type": "function_call",
                         "call_id": dynamic_call_id,
+                        "namespace": "codex_app",
                         "name": tool_name,
                         "arguments": tool_call_arguments,
                     }
@@ -754,6 +755,7 @@ async fn tool_search_returns_deferred_dynamic_tool_and_routes_follow_up_call() -
         "additionalProperties": false,
     });
     let dynamic_tool = DynamicToolSpec {
+        namespace: Some("codex_app".to_string()),
         name: tool_name.to_string(),
         description: tool_description.to_string(),
         input_schema: input_schema.clone(),
@@ -793,6 +795,7 @@ async fn tool_search_returns_deferred_dynamic_tool_and_routes_follow_up_call() -
         unreachable!("event guard guarantees DynamicToolCallRequest");
     };
     assert_eq!(request.call_id, dynamic_call_id);
+    assert_eq!(request.namespace.as_deref(), Some("codex_app"));
     assert_eq!(request.tool, tool_name);
     assert_eq!(request.arguments, tool_args);
 
@@ -832,12 +835,17 @@ async fn tool_search_returns_deferred_dynamic_tool_and_routes_follow_up_call() -
     assert_eq!(
         tools,
         vec![json!({
-            "type": "function",
-            "name": tool_name,
-            "description": tool_description,
-            "strict": false,
-            "defer_loading": true,
-            "parameters": input_schema,
+            "type": "namespace",
+            "name": "codex_app",
+            "description": "Tools in the codex_app namespace.",
+            "tools": [{
+                "type": "function",
+                "name": tool_name,
+                "description": tool_description,
+                "strict": false,
+                "defer_loading": true,
+                "parameters": input_schema,
+            }],
         })]
     );