ci: route backend/cpp/ds4/ changes through changed-backends.js

scripts/changed-backends.js:inferBackendPath has an explicit branch per
cpp dockerfile suffix (ik-llama-cpp, turboquant, llama-cpp). Without a
matching branch the function returns null, the backend never lands in
the path map, and PR change-detection cannot map "backend/cpp/ds4/X
changed" -> "rebuild ds4 image".

This is why PR #9761 produced zero ds4 jobs even though it directly
edits backend/cpp/ds4/Makefile.

Adds the missing branch (Dockerfile.ds4 -> backend/cpp/ds4/), placed
before the llama-cpp branch (since both share the .cpp ancestry but
ds4 is more specific - same ordering rule documented in
.agents/adding-backends.md).

Verified with a local Node simulation of the script against this PR's
diff: the path map now contains 'ds4 -> backend/cpp/ds4/' and a
'backend/cpp/ds4/Makefile' change correctly triggers the ds4 backend
in the rebuild set.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

Author: mudler

scripts/changed-backends.js

@@ -32,6 +32,9 @@ function inferBackendPath(item) {
     // via a thin wrapper Makefile. Changes to either dir should retrigger it.
     return `backend/cpp/turboquant/`;
   }
+  if (item.dockerfile.endsWith("ds4")) {
+    return `backend/cpp/ds4/`;
+  }
   if (item.dockerfile.endsWith("llama-cpp")) {
     return `backend/cpp/llama-cpp/`;
   }