[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/state/src/runtime/threads.rs

@@ -56,7 +56,7 @@ WHERE threads.id = ?
     ) -> anyhow::Result<Option<Vec<DynamicToolSpec>>> {
         let rows = sqlx::query(
             r#"
-SELECT name, description, input_schema, defer_loading
+SELECT namespace, name, description, input_schema, defer_loading
 FROM thread_dynamic_tools
 WHERE thread_id = ?
 ORDER BY position ASC
@@ -73,6 +73,7 @@ ORDER BY position ASC
             let input_schema: String = row.try_get("input_schema")?;
             let input_schema = serde_json::from_str::<Value>(input_schema.as_str())?;
             tools.push(DynamicToolSpec {
+                namespace: row.try_get("namespace")?,
                 name: row.try_get("name")?,
                 description: row.try_get("description")?,
                 input_schema,
@@ -778,16 +779,18 @@ ON CONFLICT(id) DO UPDATE SET
 INSERT INTO thread_dynamic_tools (
     thread_id,
     position,
+    namespace,
     name,
     description,
     input_schema,
     defer_loading
-) VALUES (?, ?, ?, ?, ?, ?)
+) VALUES (?, ?, ?, ?, ?, ?, ?)
 ON CONFLICT(thread_id, position) DO NOTHING
                 "#,
             )
             .bind(thread_id.as_str())
             .bind(position)
+            .bind(tool.namespace.as_deref())
             .bind(tool.name.as_str())
             .bind(tool.description.as_str())
             .bind(input_schema)