[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/sqlite_state.rs

@@ -103,6 +103,7 @@ async fn backfill_scans_existing_rollouts() -> Result<()> {
 
     let dynamic_tools = vec![
         DynamicToolSpec {
+            namespace: Some("codex_app".to_string()),
             name: "geo_lookup".to_string(),
             description: "lookup a city".to_string(),
             input_schema: json!({
@@ -113,6 +114,7 @@ async fn backfill_scans_existing_rollouts() -> Result<()> {
             defer_loading: true,
         },
         DynamicToolSpec {
+            namespace: None,
             name: "weather_lookup".to_string(),
             description: "lookup weather".to_string(),
             input_schema: json!({