fix(cookbook): diagnose 'no GGUF file' serve failures clearly (#811) (#866)

When serving with the llama.cpp backend and no .gguf file exists on the host,
the GGUF launcher prelude exits with 'ERROR: No GGUF found on this host', but
_diagnose_serve_output had no matching pattern, so the UI showed a generic
crash instead of explaining the cause. Add a diagnosis pattern for the
no-GGUF case so users are told a .gguf is required and pointed at downloading
a GGUF build, instead of an opaque crash.

Closes #811

routes/cookbook_routes.py

@@ -163,6 +163,11 @@ def setup_cookbook_routes() -> APIRouter:
                 "llama.cpp / llama-cpp-python dependencies are missing.",
                 [{"label": "install llama.cpp dependencies or llama-cpp-python[server]", "op": "dependency", "package": "llama-cpp-python[server]"}],
             ),
+            (
+                r"No GGUF found on this host|no \.gguf file|No GGUF file found",
+                "No GGUF file found for this model on this host. The llama.cpp backend needs a .gguf file.",
+                [{"label": "download a GGUF build of this model (repo name usually ends in -GGUF, file like Q4_K_M.gguf)", "op": "manual"}],
+            ),
             (
                 r"No module named 'torch'|No module named torch|No module named 'diffusers'|No module named diffusers",
                 "Diffusion serving requires PyTorch and diffusers.",