refactor(python): consolidate tests and focus on Python logic

- Remove test_altair_e2e.py (e2e naming no longer meaningful)
- Consolidate all tests into test_ggsql.py
- Focus tests on Python-specific logic:
  - DataFrame conversion via narwhals
  - Writer parameter validation
  - Return type handling
- Remove tests that were testing Rust logic (chart structure, encodings, etc.)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cpsievert