fix(ci): re-validate author association via REST in close-invalid-publish-prs (#1440)

## What

Stage 2 of the auto-close automation (`close-invalid-publish-prs.yml`)
re-derived the PR author association with:

```bash
gh pr view "$PR_NUMBER" --json authorAssociation --jq '.authorAssociation'
```

But `gh pr view --json` has **no** `authorAssociation` field. `gh` exits
non-zero with `Unknown JSON field: "authorAssociation"`, and because the
step runs under `set -euo pipefail`, the whole job aborts **before** it
comments, labels, or closes. Flagged PRs therefore stayed open.

## Why it went unnoticed

- **Stage 1 (detect) was fine** — it reads `author_association` from the
event payload (`github.event.pull_request.author_association`), which is
valid.
- Only the **stage-2 re-derivation** was broken.
- Every prior stage-2 run showed "success" because those PRs weren't
flagged: with no artifact, stage 2 exits early at the artifact check and
never reaches the broken line.
- **[PR
#1438](https://github.com/modelcontextprotocol/registry/pull/1438) was
the first genuine publish-attempt PR** since the automation landed
(#1393), so it was the first to exercise the bug — [run
`29178568034`](https://github.com/modelcontextprotocol/registry/actions/runs/29178568034)
failed on exactly this line.

## Fix

Fetch `author_association` from the REST pulls endpoint, which does
expose it (verified: returns `FIRST_TIME_CONTRIBUTOR` for #1438):

```bash
gh api "repos/$GH_REPO/pulls/$PR_NUMBER" --jq '.author_association'
```

Both `$GH_REPO` and `$PR_NUMBER` are already-sanitized env vars, so no
new injection surface.

## Follow-up

Stage 2 is idempotent (guarded by the `invalid` label), so once merged a
maintainer can re-run it — or simply re-open/re-close — to clean up
#1438.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/close-invalid-publish-prs.yml

@@ -95,7 +95,10 @@ jobs:
             exit 0
           fi
 
-          assoc="$(gh pr view "$PR_NUMBER" --json authorAssociation --jq '.authorAssociation')"
+          # Use the REST endpoint: `gh pr view --json` does not expose an
+          # authorAssociation field, so calling it there exits non-zero and,
+          # under `set -euo pipefail`, aborts the job before we ever close.
+          assoc="$(gh api "repos/$GH_REPO/pulls/$PR_NUMBER" --jq '.author_association')"
           case "$assoc" in
             MEMBER|OWNER|COLLABORATOR)
               echo "Author association '$assoc' is trusted; skipping."