Use shared Python version check action (#26271)

## Summary

- Replace the duplicated weekly Python version checker and shell
issue-creation steps with the shared action from
[microsoft/vscode-python-ci#4](microsoft/vscode-python-ci#4),
pinned to `c1aa1190277c85c93ab142e8909bcd11453ce30b`.
- Supply explicit workflow paths and Python matrix keys.
- Preserve the Monday schedule, manual dispatch, checkout pin,
repository-owner guard, permissions, concurrency, and `debt` /
`python-version` issue labels.
- Remove the obsolete local checker and its tests; the shared action
owns the implementation and unit tests.

## Validation

- Prettier passes for the changed workflow; YAML parsing and `git diff
--check` pass.
- Exercised the pinned checker in check-only mode against the actual
configured workflows using Python.org release data.
- Verified coverage matches the previous checker across all seven
migrations: 14 workflows and 19 explicit matrices.
- Verified issue creation arguments, repository, labels, and token
forwarding with a mocked subprocess. No live issues were created.
- The shared action's post-merge CI passed.

## Behavior change

The shared action intentionally defers duplicate-issue detection. Issue
creation remains enabled, so each scheduled/manual run with missing
coverage creates an issue even if one already exists or was previously
closed. Concurrency prevents overlapping runs, not duplicates across
runs.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 956d3afd-ae0b-486c-a60c-3a6276c23213

Author: edvilme

.github/workflows/check-python-versions.yml

@@ -19,42 +19,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
-      - uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
+      - name: Check supported Python versions
+        uses: microsoft/vscode-python-ci/actions/check-python-versions@c1aa1190277c85c93ab142e8909bcd11453ce30b
         with:
-          python-version: '3.15'
-      - name: Check for a new Python version
-        id: check
-        shell: bash
-        run: |
-          version="$(python build/update_python_versions.py)"
-          echo "version=$version" >> "$GITHUB_OUTPUT"
-      - name: Create issue
-        if: steps.check.outputs.version != ''
-        env:
-          GH_TOKEN: ${{ github.token }}
-          VERSION: ${{ steps.check.outputs.version }}
-        shell: bash
-        run: |
-          title="CI Python Versions: Update to Python $VERSION"
-          issue_number="$(
-            gh api --paginate \
-              -H "X-GitHub-Api-Version: 2022-11-28" \
-              "/repos/$GITHUB_REPOSITORY/issues?state=all&per_page=100" \
-              --jq ".[] | select(has(\"pull_request\") | not) | select(.title == \"$title\") | .number"
-          )"
-          if [[ -z "$issue_number" ]]; then
-            gh issue create \
-              --title "$title" \
-              --label debt \
-              --label python-version \
-              --body "Python $VERSION has a published release candidate according to the [Python.org downloads API](https://www.python.org/api/v2/downloads/release/?is_published=true&pre_release=true&version=3), but is missing from at least one CI test matrix.
-
-          ### Requested changes
-          - [ ] Ensure every explicit-version \`python\` matrix in \`.github/workflows/pr-check.yml\` includes Python $VERSION.
-          - [ ] Ensure every explicit-version \`python\` matrix in \`.github/workflows/build.yml\` includes Python $VERSION.
-          - [ ] Run the tests on Python $VERSION and resolve any compatibility failures.
-
-          Compare matrix entries by major.minor; patch and prerelease/dev variants of $VERSION already count as coverage. Add the version only where missing and preserve existing entries. Floating \`3.x\` entries do not declare release-candidate coverage.
-
-          Keep minimum-supported-version and build-runtime Python pins unchanged; adding CI coverage does not require raising the minimum supported Python version."
-          fi
+          files: |
+            - path: .github/workflows/pr-check.yml
+              key: python
+            - path: .github/workflows/build.yml
+              key: python
+          token: ${{ github.token }}
+          issue-labels: debt,python-version