refactor(ci): consolidate test-plugin and nightly into single workflow

Merge test-plugin-nightly.yml into test-plugin.yml using a matrix
include entry. The nightly build (PHP 8.2, WP nightly) now runs
alongside the standard PHP × WP matrix as a single workflow.

This eliminates the near-duplicate workflow file and makes it easier
to maintain the test matrix in one place.

Relates to #411, #374

Author: josephfusco

.github/workflows/test-plugin-nightly.yml

@@ -13,8 +13,13 @@ jobs:
       matrix:
         php: [ '8.3', '8.1' ]
         wordpress: [ '6.9', '6.8', '6.7', '6.6' ]
+        wp-nightly: [ false ]
+        include:
+          - php: '8.2'
+            wordpress: '6.9'
+            wp-nightly: true
       fail-fast: false
-    name: WordPress ${{ matrix.wordpress }}, PHP ${{ matrix.php }}
+    name: WordPress ${{ matrix.wp-nightly && 'Nightly' || matrix.wordpress }}, PHP ${{ matrix.php }}
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -42,6 +47,12 @@ jobs:
         working-directory: ./
         run: docker exec -e COVERAGE=1 $(docker compose ps -q wordpress) init-testing-environment.sh
 
+      - name: Upgrade to WordPress nightly
+        if: matrix.wp-nightly
+        run: |
+          docker exec --workdir=/var/www/html/wp-content/plugins/wp-graphql-content-blocks \
+            $(docker compose ps -q wordpress) wp core upgrade --version=nightly --force --allow-root
+
       - name: Install and activate WP GraphQL
         working-directory: ./
         run: docker exec --workdir=/var/www/html/wp-content/plugins/wp-graphql-content-blocks $(docker compose ps -q wordpress) wp plugin install wp-graphql --activate --allow-root