[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/README.md

@@ -16,7 +16,7 @@ schema and Responses API tool primitives that no longer need to live in
 - `ResponsesApiTool`
 - `FreeformTool`
 - `FreeformToolFormat`
-- `ToolSearchOutputTool`
+- `LoadableToolSpec`
 - `ResponsesApiWebSearchFilters`
 - `ResponsesApiWebSearchUserLocation`
 - `ResponsesApiNamespace`
@@ -35,6 +35,7 @@ schema and Responses API tool primitives that no longer need to live in
 - `create_tools_json_for_responses_api()`
 - `mcp_call_tool_result_output_schema()`
 - `tool_definition_to_responses_api_tool()`
+- `dynamic_tool_to_loadable_tool_spec()`
 - `dynamic_tool_to_responses_api_tool()`
 - `mcp_tool_to_responses_api_tool()`
 - `mcp_tool_to_deferred_responses_api_tool()`