ci: extract extension test merge into reusable composite action

Used in both test-smokes-parallel.yml (for bucket discovery) and
test-smokes.yml (for each test runner job's fresh checkout).

Author: jkrumbiegel

.github/actions/merge-extension-tests/action.yml

@@ -0,0 +1,16 @@
+name: "Merge extension tests into test tree"
+description: >
+  Copies test files from extension subtrees into the main test directories
+  so they are discovered by the test runner and bucket creation.
+  Extension tests live in their own repos (single source of truth) and are
+  pulled in via git subtree — this step avoids duplicating them.
+
+runs:
+  using: "composite"
+  steps:
+    - name: Merge julia-engine tests
+      shell: bash
+      run: |
+        SUBTREE=src/resources/extension-subtrees/julia-engine/tests
+        cp -r "$SUBTREE/docs/julia-engine" tests/docs/julia-engine
+        cp -r "$SUBTREE/smoke/julia-engine" tests/smoke/julia-engine

.github/workflows/test-smokes-parallel.yml

@@ -60,6 +60,8 @@ jobs:
         with:
           version: ${{ github.ref == 'refs/heads/main' && 'pre-release' || 'release' }}
 
+      - uses: ./.github/actions/merge-extension-tests
+
       - name: Create Job for tests
         id: tests-buckets
         run: |

.github/workflows/test-smokes.yml

@@ -241,6 +241,8 @@ jobs:
           echo "Julia Jupyter:"
           uv run julia --project=. -e "import IJulia;println(IJulia.JUPYTER);println(IJulia.find_jupyter_subcommand(\"notebook\"))"
 
+      - uses: ./.github/actions/merge-extension-tests
+
       - name: Setup timing file for timed test
         if: ${{ matrix.time-test == true }}
         run: |