build: collect per-test timing data via --junit-xml

Adds --junit-xml to the pytest invocation so each shard emits a junit
file with per-test durations. The file is uploaded as a CI artifact so
timing data can be downloaded and used to drive optimal shard rebalancing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: feanil

.github/workflows/unit-tests.yml

@@ -119,7 +119,16 @@ jobs:
       - name: run tests
         shell: bash
         run: |
-          python -Wd -m pytest -p no:randomly --ds=${{ env.settings_path }} ${{ env.unit_test_paths }} --cov=.
+          python -Wd -m pytest -p no:randomly --ds=${{ env.settings_path }} ${{ env.unit_test_paths }} --cov=. \
+            --junit-xml=reports/junit-${{ matrix.shard_name }}.xml
+
+      - name: Upload pytest timing report
+        if: always()
+        uses: actions/upload-artifact@v7
+        with:
+          name: junit-${{ matrix.shard_name }}-${{ matrix.python-version }}-${{ matrix.django-version }}-${{ matrix.mongo-version }}-${{ matrix.os-version }}
+          path: reports/junit-${{ matrix.shard_name }}.xml
+          overwrite: true
 
       - name: rename warnings json file
         if: success()