refactor(cookbook): move endpoint registration out of the routes closure (#7031)

* refactor(cookbook): move endpoint registration out of the routes closure

_auto_register_llm_endpoint and _auto_register_image_endpoint use no
closure state and model_serve is their only caller, so they can live in a
plain module and be imported and tested directly. Bodies are unchanged
apart from indentation.

Tests now import the helpers instead of reading them from the route's
closure. That lookup also failed if the route stopped referencing a
helper, so add a test that pins which helper model_serve calls.

Part of #4071

* fix(cookbook): bound the OLLAMA_HOST port scan

The pattern rescanned the rest of the token from every OLLAMA_HOST=
occurrence, so a long serve command with repeated assignments took
quadratic time. model_serve runs this on the event loop, and a 192 KB
command stalled the whole app for about 9s.

Cap the host at 253 characters, the longest DNS name, so every valid host
still matches.

Part of #4071

tests/test_cookbook_serve_lifecycle.py

@@ -156,28 +156,10 @@ async def test_scheduled_stop_requires_session_identity(endpoint_db, endpoint_ht
 
 
 def _launch_image(repo_id, port, serve_session_id):
-    import routes.cookbook_routes as cookbook_routes
+    from routes.cookbook.endpoint_registration import _auto_register_image_endpoint
     from routes.cookbook_helpers import ServeRequest
 
-    router = cookbook_routes.setup_cookbook_routes()
-
-    serve = next(
-        route.endpoint
-        for route in router.routes
-        if route.path == "/api/model/serve"
-        and "POST" in route.methods
-    )
-
-    cells = dict(
-        zip(
-            serve.__code__.co_freevars,
-            serve.__closure__ or (),
-        )
-    )
-
-    register = cells["_auto_register_image_endpoint"].cell_contents
-
-    endpoint_id = register(
+    endpoint_id = _auto_register_image_endpoint(
         ServeRequest(
             repo_id=repo_id,
             cmd=f"python scripts/diffusion_server.py --port {port}",