perf: run TestSuiteGate pytest from project root with correct interpreter

[omsherikar]

Summary

TestSuiteGate invokes python3 -m pytest with cwd=str(file_path.parent) (the changed file directory), not project_root. That can break collection (conftest.py, pytest.ini, pyproject.toml [tool.pytest], PYTHONPATH) and forces odd layout assumptions. It also hard-codes python3 instead of sys.executable, which can mismatch the environment running Refactron.

Suggested direction

• Use sys.executable and set cwd to project_root when available; fall back safely when project_root is None.
• Optionally pass through pytest args from config (markers, timeout) to avoid redundant subprocess cold starts where possible.

Acceptance

• Pytest runs with the same interpreter as the host process and discovers config from repo root in typical layouts.
• Existing tests updated or added to cover root vs nested file paths.

Code

• refactron/verification/checks/test_gate.py — subprocess.run / cmd construction