[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/tools/src/responses_api_tests.rs

@@ -1,7 +1,7 @@
+use super::LoadableToolSpec;
 use super::ResponsesApiNamespace;
 use super::ResponsesApiNamespaceTool;
 use super::ResponsesApiTool;
-use super::ToolSearchOutputTool;
 use super::dynamic_tool_to_responses_api_tool;
 use super::mcp_tool_to_deferred_responses_api_tool;
 use super::tool_definition_to_responses_api_tool;
@@ -51,6 +51,7 @@ fn tool_definition_to_responses_api_tool_omits_false_defer_loading() {
 #[test]
 fn dynamic_tool_to_responses_api_tool_preserves_defer_loading() {
     let tool = DynamicToolSpec {
+        namespace: None,
         name: "lookup_order".to_string(),
         description: "Look up an order".to_string(),
         input_schema: json!({
@@ -130,8 +131,8 @@ fn mcp_tool_to_deferred_responses_api_tool_sets_defer_loading() {
 }
 
 #[test]
-fn tool_search_output_namespace_serializes_with_deferred_child_tools() {
-    let namespace = ToolSearchOutputTool::Namespace(ResponsesApiNamespace {
+fn loadable_tool_spec_namespace_serializes_with_deferred_child_tools() {
+    let namespace = LoadableToolSpec::Namespace(ResponsesApiNamespace {
         name: "mcp__codex_apps__calendar".to_string(),
         description: "Plan events".to_string(),
         tools: vec![ResponsesApiNamespaceTool::Function(ResponsesApiTool {