chore(ci): use matrix strategy for Python versions in unit and integration test jobs

Add a matrix strategy to the unit_tests and integration_tests jobs in
ci.yml so that tests run against all Python versions listed in
SUPPORTED_PYTHONS in noxfile.py (currently ["3.13"]).

Changes:
- Add strategy.matrix.python-version to unit_tests and integration_tests
- Set fail-fast: false so all matrix versions run even if one fails
- Use matrix.python-version in the container image tag
- Invoke nox with the versioned session name (e.g. unit_tests-3.13)
- Scope uv cache keys per Python version to avoid cross-version pollution
- Remove the now-unused global PYTHON_VERSION env var

When SUPPORTED_PYTHONS is extended in noxfile.py, only the matrix list
in ci.yml needs updating to add the new version.

ISSUES CLOSED: #1539

.forgejo/workflows/ci.yml

@@ -8,7 +8,6 @@ on:
 
 env:
     UV_VERSION: "0.8.0"
-    PYTHON_VERSION: "3.13"
     NOX_DEFAULT_VENV_BACKEND: "uv"
 
 jobs:
@@ -142,8 +141,12 @@ jobs:
 
     unit_tests:
         runs-on: docker
+        strategy:
+            matrix:
+                python-version: ["3.13"]
+            fail-fast: false
         container:
-            image: python:3.13-slim
+            image: python:${{ matrix.python-version }}-slim
         steps:
             - name: Install system dependencies (nodejs for checkout, git for merge tests, curl/tar for Helm)
               run: |
@@ -172,20 +175,24 @@ jobs:
               uses: actions/cache@v3
               with:
                   path: ~/.cache/uv
-                  key: uv-${{ hashFiles('pyproject.toml') }}
+                  key: uv-tests-${{ matrix.python-version }}-${{ hashFiles('pyproject.toml') }}
                   restore-keys: |
-                      uv-
+                      uv-tests-${{ matrix.python-version }}-
 
             - name: Run unit tests via nox
               run: |
-                  nox -s unit_tests
+                  nox -s "unit_tests-${{ matrix.python-version }}"
               env:
                   NOX_DEFAULT_VENV_BACKEND: uv
 
     integration_tests:
         runs-on: docker
+        strategy:
+            matrix:
+                python-version: ["3.13"]
+            fail-fast: false
         container:
-            image: python:3.13-slim
+            image: python:${{ matrix.python-version }}-slim
         steps:
             - name: Install system dependencies (nodejs for checkout, git for integration tests, curl/tar for Helm)
               run: |
@@ -214,13 +221,13 @@ jobs:
               uses: actions/cache@v3
               with:
                   path: ~/.cache/uv
-                  key: uv-${{ hashFiles('pyproject.toml') }}
+                  key: uv-tests-${{ matrix.python-version }}-${{ hashFiles('pyproject.toml') }}
                   restore-keys: |
-                      uv-
+                      uv-tests-${{ matrix.python-version }}-
 
             - name: Run integration tests via nox
               run: |
-                  nox -s integration_tests
+                  nox -s "integration_tests-${{ matrix.python-version }}"
               env:
                   NOX_DEFAULT_VENV_BACKEND: uv
                   CLEVERAGENTS_REQUIRE_HELM_RENDER_ASSERTIONS: "true"