[TE-5274] Add pytest 9.x CI matrix entry for subtest integration tests

Add a sixth matrix entry (Python 3.13 + pytest 9) so the 5 integration
tests in test_integration_subtests.py are actually exercised in CI.
Those tests require pytest >= 9.0 for built-in SubtestReport support,
but uv.lock pins pytest to 8.4.1 (pytest 9 dropped Python 3.9).

Uses uv run --with 'pytest>=9' rather than uv pip install because
uv run syncs the environment to uv.lock before execution, which would
silently revert a pip-installed override. The --with flag creates an
ephemeral overlay that persists through the run, including subprocesses
spawned via sys.executable.

Author: gchan

.github/workflows/python.yaml

@@ -14,6 +14,10 @@ jobs:
     strategy:
       matrix:
         python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
+        uv-run-flags: [""]
+        include:
+          - python-version: "3.13"
+            uv-run-flags: "--with 'pytest>=9'"
 
     env:
       BUILDKITE_ANALYTICS_TOKEN: ${{ secrets.BUILDKITE_ANALYTICS_TOKEN }}
@@ -29,7 +33,7 @@ jobs:
       - name: Install dependencies
         run: uv sync --all-extras
       - name: Run tests
-        run: uv run pytest
+        run: uv run ${{ matrix.uv-run-flags }} pytest
 
   pylint:
     runs-on: ubuntu-latest