chore: Update python-version in .github/workflows/python-unit-tests.yml to ["3.9", "3.10", "3.11", "3.12", "3.13"]

Verified with uv on Python 3.12 and 3.13 using the same ignores as CI.
3.12:
uv python install 3.12 && uv venv --python 3.12 .venv && source .venv/bin/activate
uv sync --extra test --extra eval --extra a2a
python -m pytest tests/unittests --ignore=tests/unittests/artifacts/test_artifact_service.py --ignore=tests/unittests/tools/google_api_tool/test_googleapi_to_openapi_converter.py
3.13: repeated the above with 3.13 (separate env)
Result: All unit tests passed

PiperOrigin-RevId: 794669437

Author: George Weale

.github/workflows/python-unit-tests.yml

@@ -25,7 +25,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.9", "3.10", "3.11"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
 
     steps:
     - name: Checkout code