Fix CI test failures by adding matplotlib backend and nltk punkt

  - Set the MPLBACKEND environment variable to Agg for non-interactive test environment
  - Explicitly download nltk punkt tokenizer needed for text visualizers
  - These changes resolve test failures in the CI pipeline

Author: lwgray

.github/workflows/ci.yml

@@ -38,11 +38,13 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install -r tests/requirements.txt
-        python -m nltk.downloader popular
+        python -m nltk.downloader popular punkt
 
     - name: Run Unit Tests
       run: |
         make test
+        env:
+          MPLBACKEND: Agg
 
     - name: Upload Coverage
       uses: codecov/codecov-action@v1