fix(ci/backend): Use Poetry version from lockfile (#9729)

Currently, our CI always uses the latest version of Poetry. This causes
issues with the lockfile check whenever a new Poetry version is
released, especially if that new version has different lockfile
generation behavior.

This new mechanism determines the Poetry version to use as follows:
- Get Poetry version from backend/poetry.lock in the current branch
- Get Poetry version from backend/poetry.lock on the base branch
- Use the newest version out of the two found versions

This way, we don't automatically update to new Poetry versions, but it
is still possible to update to newer versions through pull requests.

.github/workflows/platform-backend-ci.yml

@@ -80,18 +80,35 @@ jobs:
 
       - name: Install Poetry (Unix)
         run: |
-          curl -sSL https://install.python-poetry.org | python3 -
+          # Extract Poetry version from backend/poetry.lock
+          HEAD_POETRY_VERSION=$(head -n 1 poetry.lock | grep -oP '(?<=Poetry )[0-9]+\.[0-9]+\.[0-9]+')
+          echo "Found Poetry version ${HEAD_POETRY_VERSION} in backend/poetry.lock"
+
+          if [ -n "$BASE_REF" ]; then
+            BASE_BRANCH=${BASE_REF/refs\/heads\//}
+            BASE_POETRY_VERSION=$((git show "origin/$BASE_BRANCH":./poetry.lock; true) | head -n 1 | grep -oP '(?<=Poetry )[0-9]+\.[0-9]+\.[0-9]+')
+            echo "Found Poetry version ${BASE_POETRY_VERSION} in backend/poetry.lock on ${BASE_REF}"
+            POETRY_VERSION=$(printf '%s\n' "$HEAD_POETRY_VERSION" "$BASE_POETRY_VERSION" | sort -V | tail -n1)
+          else
+            POETRY_VERSION=$HEAD_POETRY_VERSION
+          fi
+          echo "Using Poetry version ${POETRY_VERSION}"
+
+          # Install Poetry
+          curl -sSL https://install.python-poetry.org | POETRY_VERSION=$POETRY_VERSION python3 -
 
           if [ "${{ runner.os }}" = "macOS" ]; then
             PATH="$HOME/.local/bin:$PATH"
             echo "$HOME/.local/bin" >> $GITHUB_PATH
           fi
+        env:
+          BASE_REF: ${{ github.base_ref || github.event.merge_group.base_ref }}
 
       - name: Check poetry.lock
         run: |
           poetry lock
 
-          if ! git diff --quiet poetry.lock; then
+          if ! git diff --quiet --ignore-matching-lines="^# " poetry.lock; then
             echo "Error: poetry.lock not up to date."
             echo
             git diff poetry.lock