pyproject, github/workflows: Add diff-cover for CI checks and dependencies

- Added `diff-cover` (>=10.2.0) as a development dependency in `pyproject.toml`.
- Modified `test-and-publish.yml` to add a new step for running `diff-cover`
  during pull request workflows.
  - Ensures new code meets a minimum 90% coverage threshold.
  - Generates a Markdown report for coverage in modified code sections.

Assisted-by: Codex

Author: datalogics-kam

.github/workflows/test-and-publish.yml

@@ -38,6 +38,16 @@ jobs:
         run: uvx nox --python ${{ matrix.python-version }} --session tests -- --no-parallel
         env:
           PDFREST_API_KEY: ${{ secrets.PDFREST_API_KEY }}
+      - name: Fetch base branch for diff-cover
+        if: github.event_name == 'pull_request'
+        run: git fetch origin ${{ github.base_ref }} --depth=1
+      - name: Run diff-cover (new code must be >= 90%)
+        if: github.event_name == 'pull_request'
+        run: >
+          diff-cover coverage/py${{ matrix.python-version }}/coverage.xml
+          --compare-branch origin/${{ github.base_ref }}
+          --fail-under 90
+          --markdown-report coverage/py${{ matrix.python-version }}/diff-cover.md
       - name: Upload coverage reports
         if: always()
         uses: actions/upload-artifact@v4

pyproject.toml

@@ -34,6 +34,7 @@ dev = [
     "nox>=2025.5.1",
     "basedpyright>=1.34.0",
     "python-dotenv>=1.0.1",
+    "diff-cover>=10.2.0",
 ]
 
 [tool.pytest.ini_options]