ci: add cross-OS E2E tests and gracefully skip missing backends - #1635

Resolves #549
This PR updates our CI pipelines to ensure End-to-End tests are validated against all supported Operating Systems natively, rather than just Linux.

What changed:

1. GitHub Actions Workflow Matrix:

   • Updated .github/workflows/e2e-tests.yml to use a matrix strategy covering ubuntu-latest, macos-latest, and windows-latest.
   • Explicitly configured the E2E test execution step to use shell: bash. This allows the existing ./tests/run_tests.sh script to execute flawlessly on Windows runners using Git Bash.

2. Graceful Database Driver Fallbacks:

   • Modified test_verify_databases_parity.py to dynamically detect which database drivers are available on the current OS (falkordblite, kuzu, etc.).
   • Parity tests now gracefully skip unsupported backends instead of crashing with StopIteration or KeyError on systems where they aren't installed (e.g., falkordblite on Windows).
   • The parity results table dynamically updates its columns based solely on the available drivers.

How to verify:

• Push to this branch and observe the GitHub Actions tab.
• Verify that the End-to-end Tests workflow spawns three separate jobs for Ubuntu, macOS, and Windows.
• Confirm all three jobs pass successfully.

📸 Screenshots (CI Results):