server: use the moved test model in the router CORS test (#32117)
test_router_restricted_cors_same_origin_reaches_child still named
ggml-org/tinygemma3-GGUF, which the router no longer finds since the test
models moved to ggml-org-dev, so the Server test job fails on master.
Assisted-by: Claude (Opus 5.5 Medium)