fix(ci): rebuild backends when shared build inputs change (#10975)

The backend matrix path filter only matched files under a backend's own
directory, so a change to shared build infrastructure rebuilt nothing at
all: an empty matrix, every job green, and the change reaching no image.

PR #10946 fixed scripts/build/package-gpu-libs.sh shipping a partial
4-of-8 cuDNN library set, which mixed versions with the venv's pip cuDNN
and produced CUDNN_STATUS_SUBLIBRARY_VERSION_MISMATCH at inference time.
It merged 1h48m after the weekly full-matrix cron had already run, so no
backend image ever received the fix and nothing signalled that it had
been un-shipped.

Add a SHARED_BUILD_INPUTS table mapping each shared path to the narrowest
set of matrix entries it can honestly invalidate, plus a generic rule for
backend/Dockerfile.<x> (which each entry already names). A full matrix is
417 Linux + 56 Darwin builds, so package-gpu-libs.sh now rebuilds the 176
Python entries rather than everything. Unclassified files under
scripts/build/ fall back to a full rebuild deliberately: over-building is
recoverable, silently shipping nothing is not.

Extract the filtering logic to scripts/lib/backend-filter.mjs so it can be
unit-tested without bun, js-yaml or a GitHub API round-trip, and run those
tests from the existing lint workflow via `make test-ci-scripts`.


Assisted-by: Claude Code:claude-opus-4-8[1m] [Read] [Edit] [Bash]

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

.github/backend-matrix.yml

@@ -23,7 +23,7 @@
 # checklist in .agents/adding-backends.md (includeDarwin entry, the index.yaml
 # `metal:` capability + `metal-<backend>` image entries, a `run.sh` Darwin/DYLD
 # branch for C/C++ backends, and the inferBackendPathDarwin case in
-# scripts/changed-backends.js so the path filter actually builds it).
+# scripts/lib/backend-filter.mjs so the path filter actually builds it).
 
 # Linux matrix (consumed by backend-jobs).
 include:

.github/workflows/lint.yml

@@ -57,3 +57,12 @@ jobs:
       - uses: actions/checkout@v7
       - name: run packaging script tests
         run: make test-build-scripts
+
+      # The backend matrix path filter fails silently: a miss emits an empty
+      # matrix, every job goes green, and the change reaches no image (#10946).
+      # Its tests need only node, so they ride along with this job.
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+      - name: run CI script tests
+        run: make test-ci-scripts