Fix dependency review evidence failure on large pull requests #2485

Observed failure

On grouped dependency update PR #2479, the dependency review action reported 0 vulnerable packages and 0 incompatible or invalid licenses. The following evidence step failed with Maximum object size exceeded while expanding the full dependency-change output into a workflow environment variable; the artifact upload then found no file. Failed run.

Expected behavior

Keep the existing vulnerability and license policy, and save the complete dependency diff for the exact base and head commits even on large grouped updates. A missing diff must remain a failed check.

Candidate fix and validation

Fetch the diff from GitHub's dependency review compare API after the review action. Store the complete raw response with commit IDs, review outcome, count, and a digest as an artifact, without expanding the large action output in the workflow template. The focused workflow tests (14), lint, and a live read of the 1,390-change diff from #2479 passed locally. Hosted workflow validation remains pending.