Add debug Python environment details to pytest validation in run_all.py

[MobyDickDick]

Motivation

• Provide visibility into the exact Python environment used to run tests to help diagnose No module named pytest and similar import errors.
• Surface interpreter metadata such as sys.executable, sys.prefix, sys.base_prefix, PYTHONHOME, PYTHONPATH, VIRTUAL_ENV, and sys.path for debugging.
• Keep these diagnostics off by default and enable them only when TINYLANGUAGE_DEBUG_PYTEST=1 to avoid noisy output during normal runs.
• Make it easier to reproduce and investigate environment-related failures from the run_all.py runner and VS Code debug profile.

Description

• Add a debug-only environment diagnostics step to src/run_all.py that runs PYTEST_PYTHON -c "..." to print interpreter and environment details when validating pytest availability.
• Print both stdout and stderr from the diagnostics command so the full output is visible in the runner output.
• Keep diagnostics conditional on the TINYLANGUAGE_DEBUG_PYTEST environment variable and reuse the resolved repository interpreter stored in PYTEST_PYTHON.
• Update VS Code config to help enable the debug path by setting TINYLANGUAGE_DEBUG_PYTEST in the Python: run_all.py launch profile and prefer the repository venv in settings.json.

Testing

• No automated tests were executed for these changes.

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Codex Task