fix(ci): skip release-drafter on pull_request events

The release-drafter action runs on both push (main) and pull_request
events. When triggered by a PR, it attempts to update the draft release
with the PR merge ref (refs/pull/N/merge) as target_commitish, which the
GitHub Releases API rejects with a 422 validation error.

Fix: add  condition to the job so the
draft release is only updated when commits land on main. The PR trigger
is still listed in the workflow triggers for the autolabeler feature —
it will simply be skipped at the job level.

Fixes recurring failures: runs 23481318939, 23369773059, 23369768998,
23111592904 (all 'invalid target_commitish' errors).

Author: Ritchie

.github/workflows/release-drafter.yml

@@ -18,6 +18,11 @@ permissions:
 
 jobs:
   update_release_draft:
+    # Only update the release draft on push to main.
+    # On pull_request events the action would try to set target_commitish to
+    # the PR merge ref (refs/pull/N/merge), which the GitHub Releases API
+    # rejects with a 422 "Validation Failed: invalid target_commitish" error.
+    if: github.event_name == 'push'
     permissions:
       # write permission is required to create a github release
       contents: write