[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-protocol/schema/json/DynamicToolCallParams.json

@@ -5,6 +5,12 @@
     "callId": {
       "type": "string"
     },
+    "namespace": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
     "threadId": {
       "type": "string"
     },
@@ -24,4 +30,4 @@
   ],
   "title": "DynamicToolCallParams",
   "type": "object"
-}
+}