Enable PR number validation against changelog fragment in GHA

Author: carbolymer

.changes/20260413_093442_cardano-api_mateusz.galazyn_make_herald_validation_mandatory.yml

@@ -0,0 +1,5 @@
+description: Enable PR number validation in changelog fragments
+kind:
+- maintenance
+pr: 1183
+project: cardano-api

.github/workflows/check-pr-changelog.yml

@@ -28,6 +28,3 @@ jobs:
             accept-flake-config = true
 
       - uses: input-output-hk/cardano-dev/herald/.github/actions/validate@main
-        with:
-          # TODO: re-enable after backfill PR #1164 merges
-          pr: false