ci(image-scan): match Python packages against CPE data (#34136)

grype defaults match.python.using-cpes to false, so PyPI packages are
matched only against the GitHub Advisory Database. When a CVE is
published to NVD but its GHSA has not propagated to the global advisory
database, the scan reports clean even though grype's own database
already carries the NVD record with the correct version ranges.

The pypdf CVEs (CVE-2026-59935 / 59936 / 59937 / 59938, analyzed in NVD
since 2026-07-08) are the case that exposed this; their GHSA IDs are
still repo-level and return 404 from the global advisory API, so the
ecosystem matcher has nothing to match on.

Enabling CPE matching for Python closes that gap. Measured against a
v1.91.1 build the finding count goes from 28 to 38; the additions are
mostly actionable, and the few cross-product CPE collisions cannot fail
the build because --only-fixed drops the ones carrying no fix version
and the remainder land below the --fail-on high threshold.

.github/workflows/image-scan.yml

@@ -58,6 +58,8 @@ jobs:
       # free OSS, run as a pinned, checksum-verified binary; no GitHub Action
       # dependency and no vendor SaaS callout.
       - name: Scan image for fixable HIGH/CRITICAL CVEs
+        env:
+          GRYPE_MATCH_PYTHON_USING_CPES: "true"
         run: |
           "$RUNNER_TEMP/grype" litellm-image-scan:${{ github.sha }} \
             --only-fixed \