[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/app-server/tests/suite/v2/thread_unsubscribe.rs

@@ -127,6 +127,7 @@ async fn thread_unsubscribe_during_turn_keeps_turn_running() -> Result<()> {
         .send_thread_start_request(ThreadStartParams {
             model: Some("mock-model".to_string()),
             dynamic_tools: Some(vec![DynamicToolSpec {
+                namespace: None,
                 name: tool_name.to_string(),
                 description: "Deterministic wait tool".to_string(),
                 input_schema: json!({
@@ -194,6 +195,7 @@ async fn thread_unsubscribe_during_turn_keeps_turn_running() -> Result<()> {
             thread_id: thread_id.clone(),
             turn_id: started.turn_id,
             call_id: call_id.to_string(),
+            namespace: None,
             tool: tool_name.to_string(),
             arguments: tool_args,
         }