Resolve the WordPress test matrix from wordpress.org at run time

.github/workflows/tests.yml

@@ -4,11 +4,40 @@ on:
   push:
     branches: [main]
   pull_request:
+  schedule:
+    # Weekly, so new WordPress releases and trunk get exercised even when the repo is quiet.
+    - cron: '0 6 * * 1'
+  workflow_dispatch:
 
 jobs:
+  wp-versions:
+    name: Resolve WordPress versions
+    runs-on: ubuntu-latest
+    outputs:
+      branches: ${{ steps.resolve.outputs.branches }}
+    steps:
+      - name: Fetch supported branches from wordpress.org
+        id: resolve
+        run: |
+          # stable-check lists every x.y.z release. Reduce to x.y branches and keep the newest five,
+          # so the matrix picks up new majors automatically and drops old ones. Trunk rides along
+          # as "nightly" so the next release is tested before it ships.
+          branches=$(curl -sf https://api.wordpress.org/core/stable-check/1.0/ | python3 -c '
+          import json, sys
+          versions = json.load(sys.stdin)
+          branches = {".".join(v.split(".")[:2]) for v in versions}
+          branches = sorted(branches, key=lambda b: tuple(int(n) for n in b.split(".")))
+          print(json.dumps(branches[-5:] + ["nightly"]))
+          ')
+          echo "Testing against WordPress: $branches"
+          echo "branches=$branches" >> "$GITHUB_OUTPUT"
+
   phpunit:
     name: PHP ${{ matrix.php }} / WP ${{ matrix.wp }}
     runs-on: ubuntu-latest
+    needs: wp-versions
+    # Trunk is allowed to fail without failing the run.
+    continue-on-error: ${{ matrix.wp == 'nightly' }}
 
     services:
       mysql:
@@ -28,7 +57,7 @@ jobs:
       fail-fast: false
       matrix:
         php: ['7.4', '8.0', '8.1', '8.2', '8.3', '8.4', '8.5']
-        wp: ['6.5', '6.6', '6.7', '6.8', '6.9']
+        wp: ${{ fromJSON(needs.wp-versions.outputs.branches) }}
 
     steps:
       - uses: actions/checkout@v7