ci: add Ruby 4.0 to the test matrix

Ruby 4.0 was released in late 2025; the gem already runs cleanly on it
locally (rubocop and the full RSpec suite pass on 4.0.2). Adding it to
the CI matrix so regressions on Ruby 4 are caught before release.

The gem's required Ruby is `>= 2.7` and is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erimicel

.github/workflows/tests.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['3.0', '3.2', '3.3', '3.4']
+        ruby-version: ['3.0', '3.2', '3.3', '3.4', '4.0']
     services:
       typesense:
         image: typesense/typesense:30.0.alpha1