fix: bake promtool into ci-full image, add download timeout, speed up tests

- Add promtool to ci-full Dockerfile so it's pre-installed in the
  runner image instead of downloaded at runtime on every infra CI run
- Add 60s timeout to _download() in install_tools.py to prevent
  indefinite hangs when GitHub release downloads stall
- Use -n 8 workers for test-unit target (3s vs 4s with auto/16)
- Update test_download to mock urlopen instead of urlretrieve

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docker/ci-full/Dockerfile

@@ -20,5 +20,6 @@ COPY . /tmp/devx
 RUN pip install --no-cache-dir /tmp/devx[release,molecule,deploy] \
     && rm -rf /tmp/devx
 
-# Install git-cliff (changelog generator for release job) and OpenTofu (for infra deploy jobs)
-RUN python3 -m devx.tools.install_tools --tool git-cliff --tool tofu
+# Install git-cliff (changelog generator for release job), OpenTofu (for infra deploy jobs),
+# and promtool (Prometheus rule validator — used by every infra CI run for alert validation)
+RUN python3 -m devx.tools.install_tools --tool git-cliff --tool tofu --tool promtool