ci: run the raku grammar tests

.github/workflows/tests.yaml

@@ -523,8 +523,8 @@ jobs:
           token: cab4ace5-4f10-4027-8b5c-d79722234571
 
   test-grammars:
-    # Currently tests lezer grammars. We could split that out into a separate
-    # job if we want when we add more.
+    # Tests the lezer & raku grammars. We could split these into separate jobs
+    # if we want when we add more.
     runs-on: ubuntu-24.04
     needs: rules
     if:
@@ -540,12 +540,28 @@ jobs:
       - name: Install dependencies
         working-directory: grammars/prql-lezer/
         run: bun install
-      - name: Build grammar
+      - name: Build lezer grammar
         working-directory: grammars/prql-lezer/
         run: bun run build
-      - name: Test grammar
+      - name: Test lezer grammar
         working-directory: grammars/prql-lezer/
         run: bun run test
+      - name: 🦋 Setup Raku
+        uses: Raku/setup-raku@v1
+      # `prove6` is the TAP harness that runs the whole `t/` directory in one
+      # command, as `grammars/raku/README.md` documents. `zef` ships with the
+      # rakudo archive that `setup-raku` installs.
+      #
+      # `--/test` skips the test suites of `App::Prove6` and its transitive
+      # deps, which `zef` would otherwise run before installing. The README
+      # recipe deliberately omits it — a human installing locally wants those
+      # to run; here they'd only turn the job red for reasons unrelated to the
+      # PRQL grammar.
+      - name: Install prove6
+        run: zef install --/test App::Prove6
+      - name: Test raku grammar
+        working-directory: grammars/raku/
+        run: prove6 --lib t/
 
   build-devcontainer:
     needs: rules