Automate merge+regenerate flow for dependency PR generated files

[Copilot]

Dependency PRs were repeatedly conflicting on generated artifacts (**/gradle.lockfile, licenses/more-licenses.md), creating an O(N²) resolve/re-run/re-approve loop. This PR makes regeneration deterministic and automated after main moves, while preserving human intervention for real source conflicts.

• Reusable merge-and-regenerate workflow

  • Added .github/workflows/reusable-regenerate-generated-files.yml (workflow_call) as the single implementation of:
    • merge origin/main into PR head
    • classify conflicts into generated vs non-generated
    • auto-resolve generated conflicts only (placeholder resolution), abort/comment on any real source conflict
    • always run:
      • ./gradlew resolveAndLockAll --write-locks
      • ./gradlew generateLicenseReport --no-build-cache
    • commit/push only when regenerated files changed
  • Guards included:
    • skip PRs whose head repo is not microsoft/ApplicationInsights-Java
    • skip when PR already contains main

• Main fan-out + on-demand orchestration

  • Added .github/workflows/regenerate-generated-files.yml:
    • triggers: push to main, issue_comment (/regen on PR), workflow_dispatch (pr_number optional)
    • on push: enumerates open dependency PRs with gh pr list ... --label dependencies, fans out with matrix (fail-fast: false, max-parallel: 3)
    • per-PR concurrency: regen-${{ matrix.pr }} with cancel-in-progress
  • Added .github/workflows/regenerate-generated-files-runner.yml for single-PR dispatch execution path.

• Refactor existing helper to avoid logic duplication

  • Updated .github/workflows/pull-request-helper.yml to call the reusable workflow, keeping Dependabot-opened-PR behavior while sharing the same regeneration core.

• Permissions and action pinning

  • Kept required PAT push behavior (PR_HELPER_GITHUB_TOKEN) so regenerated pushes retrigger CI.
  • Third-party actions remain pinned to commit SHA.
  • Reused gradle/actions/setup-gradle with cache-read-only: true.

• Docs update

  • Updated CONTRIBUTING.md to document:
    • automatic post-main regeneration behavior
    • /regen command usage
    • when manual conflict resolution is still required (bot comment on genuine non-generated conflicts)

# conflict safety valve in reusable workflow
mapfile -t conflicted < <(git diff --name-only --diff-filter=U)

for f in "${conflicted[@]}"; do
  case "$f" in
    *gradle.lockfile|licenses/more-licenses.md) generated+=("$f") ;;
    *)                                          real+=("$f") ;;
  esac
done

if [ ${#real[@]} -gt 0 ]; then
  gh pr comment "$PR_NUMBER" --repo "$REPO" --body "$comment"
  git merge --abort
  exit 0
fi