fix(infra): resolve module crash blocking external PR enforcement (#36059)

`pr-labeler.js` used `require('@actions/core')` to access GitHub Actions
logging/failure helpers, but that module is bundled inside
`actions/github-script`'s dist — it's not resolvable via Node's
`require()` from a checked-out file on disk. Two of the three call sites
were in rarely-hit error branches, so the bug was latent. The third
(`applyTierLabel`) ran unconditionally, crashing the tier-label step on
every external PR. Because the tier step runs *before* the "add external
label" step, the crash prevented the `external` label from ever being
applied — which meant `require_issue_link.yml` never triggered and
unapproved external PRs stayed open.

## Changes
- Thread the `core` object (provided by `actions/github-script` at eval
time) through `loadAndInit()` → `init()` instead of calling
`require('@actions/core')` from the checked-out script — fixes the
`MODULE_NOT_FOUND` crash on all three call sites (`ensureLabel`,
`getContributorInfo`, `applyTierLabel`)
- Add a console-based fallback in `loadAndInit` so callers that don't
need `core.setFailed` still work without passing it
- Update all 9 `loadAndInit(github, owner, repo)` call sites across
`pr_labeler.yml`, `pr_labeler_backfill.yml`, and
`tag-external-issues.yml` to pass `core`

.github/scripts/pr-labeler.js

@@ -1,7 +1,7 @@
 // Shared helpers for pr_labeler.yml and tag-external-issues.yml.
 //
 // Usage from actions/github-script (requires actions/checkout first):
-//   const { h } = require('./.github/scripts/pr-labeler.js').loadAndInit(github, owner, repo);
+//   const { h } = require('./.github/scripts/pr-labeler.js').loadAndInit(github, owner, repo, core);
 
 const fs = require('fs');
 const path = require('path');
@@ -32,7 +32,10 @@ function loadConfig() {
   return config;
 }
 
-function init(github, owner, repo, config) {
+function init(github, owner, repo, config, core) {
+  if (!core) {
+    throw new Error('init() requires a `core` parameter (e.g., from actions/github-script)');
+  }
   const {
     trustedThreshold,
     labelColor,
@@ -60,7 +63,6 @@ function init(github, owner, repo, config) {
       } catch (createErr) {
         // 422 = label created by a concurrent run between our get and create
         if (createErr.status !== 422) throw createErr;
-        const core = require('@actions/core');
         core.info(`Label "${name}" creation returned 422 (likely already exists)`);
       }
     }
@@ -196,7 +198,6 @@ function init(github, owner, repo, config) {
         mergedCount = result?.data?.total_count ?? null;
       } catch (e) {
         if (e?.status !== 422) throw e;
-        const core = require('@actions/core');
         core.warning(`Search failed for ${author}; skipping tier.`);
       }
     }
@@ -209,7 +210,6 @@ function init(github, owner, repo, config) {
   // ── Tier label resolution ───────────────────────────────────────────
 
   async function applyTierLabel(issueNumber, author, { skipNewContributor = false } = {}) {
-    const core = require('@actions/core');
     let mergedCount;
     try {
       const result = await github.rest.search.issuesAndPullRequests({
@@ -263,9 +263,9 @@ function init(github, owner, repo, config) {
   };
 }
 
-function loadAndInit(github, owner, repo) {
+function loadAndInit(github, owner, repo, core) {
   const config = loadConfig();
-  return { config, h: init(github, owner, repo, config) };
+  return { config, h: init(github, owner, repo, config, core) };
 }
 
 module.exports = { loadConfig, init, loadAndInit };