Cache split_tests pytest --collect-only results between CI runs

.github/workflows/ci.yaml

@@ -917,12 +917,49 @@ jobs:
           key: >-
             ${{ runner.os }}-${{ runner.arch }}-${{ steps.python.outputs.python-version }}-${{
             needs.info.outputs.python_cache_key }}
+      - name: Restore pytest test counts cache
+        id: cache-pytest-counts
+        uses: actions/cache/restore@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.5
+        with:
+          path: pytest_test_counts.json
+          # Primary key is a sentinel; restore-keys pick the most recent
+          # prefix match since the real (content-addressed) key isn't
+          # known until split_tests.py runs below.
+          key: >-
+            pytest-counts-${{ runner.os }}-${{ runner.arch }}-${{
+            steps.python.outputs.python-version }}-${{
+            needs.info.outputs.python_cache_key }}-restore-sentinel
+          restore-keys: |
+            pytest-counts-${{ runner.os }}-${{ runner.arch }}-${{ steps.python.outputs.python-version }}-${{ needs.info.outputs.python_cache_key }}-
       - name: Run split_tests.py
         env:
           TEST_GROUP_COUNT: ${{ needs.info.outputs.test_group_count }}
         run: |
           . venv/bin/activate
-          python -m script.split_tests ${TEST_GROUP_COUNT} tests
+          python -m script.split_tests \
+            --cache pytest_test_counts.json \
+            ${TEST_GROUP_COUNT} tests
+      - name: Hash pytest test counts cache
+        id: cache-pytest-counts-hash
+        run: |
+          echo "hash=$(sha256sum pytest_test_counts.json | cut -d' ' -f1)" \
+            >> "$GITHUB_OUTPUT"
+      - name: Save pytest test counts cache
+        # Content-addressed key: identical content reuses the same entry.
+        # Skip the save when the restore already matched that hash.
+        if: >-
+          !endsWith(
+            steps.cache-pytest-counts.outputs.cache-matched-key,
+            steps.cache-pytest-counts-hash.outputs.hash
+          )
+        uses: actions/cache/save@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.5
+        with:
+          path: pytest_test_counts.json
+          key: >-
+            pytest-counts-${{ runner.os }}-${{ runner.arch }}-${{
+            steps.python.outputs.python-version }}-${{
+            needs.info.outputs.python_cache_key }}-${{
+            steps.cache-pytest-counts-hash.outputs.hash }}
       - name: Upload pytest_buckets
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with: