ci(model-server): lint the OpenAPI spec for breaking changes

Adds a non-failing workflow to detect breaking changes in the OpenAPI
specifications.

oasdiff was chosen because:

* it provides a GitHub Action
* it's more active than other tools

Author: languitar

.github/workflows/linting.yml

@@ -10,3 +10,29 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
       - uses: pre-commit/[email protected]
+
+  openapi-breaking-changes:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+      - name: Clone repo
+        uses: actions/checkout@v4
+        with:
+          # We need the history to find the common ancestor of the PR and the target branch from which we fetch the
+          # baseline OpenAPI specifications to compare against.
+          fetch-depth: 0
+      - name: Fetch baseline OpenAPI specifications
+        run: |
+          mergeBase="$(git merge-base --fork-point "origin/${{ github.base_ref }}")"
+          echo "mergeBase: $mergeBase"
+          git restore -s "$mergeBase" -W -- model-server-openapi/specifications
+          cp -R model-server-openapi/specifications model-server-openapi/specifications-before
+          git checkout model-server-openapi/specifications
+      - name: Run oasdiff
+        id: breaking
+        uses: oasdiff/oasdiff-action/breaking@main
+        with:
+          base: 'model-server-openapi/specifications-before/*.yaml'
+          revision: 'model-server-openapi/specifications/*.yaml'
+          composed: true