fix(tools): use _INTERNAL_BASE in serve-session endpoint registration (#3675)

#3322 renamed the loopback base to _INTERNAL_BASE, but a later Cookbook
commit reintroduced one call site using the old _COOKBOOK_BASE name,
raising NameError whenever the agent registers a model endpoint for a
running serve session.

Fixes #3669

src/tool_implementations.py

@@ -2684,7 +2684,7 @@ async def _ensure_served_endpoint(
     try:
         async with httpx.AsyncClient(timeout=30) as client:
             resp = await client.post(
-                f"{_COOKBOOK_BASE}/api/model-endpoints",
+                f"{_INTERNAL_BASE}/api/model-endpoints",
                 data=payload,
                 headers=_internal_headers(),
             )