fix(ci): remove pip cache that requires pyproject.toml or requirements.txt

actions/setup-python cache: pip fails when neither file exists (project uses
setup.py only), causing the test-gate to error before any tests run.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: aeiwz

.github/workflows/python-publish.yml

@@ -45,7 +45,6 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
-          cache: pip
 
       - name: Install build tools + dev dependencies
         run: |