feat: add mock Stata runtime for CI tests

[tobiasraabe]

Summary

• Add a dedicated tests/mock_stata.py runtime that supports the small Stata command subset used by the tests.
• Switch Stata-dependent tests to use the mock only when CI is set; local runs now require a real Stata executable on PATH.
• Emit an explicit warning showing the mock shim and module path when the mock runtime is active.
• Update generated task snippets and README examples to the current pytask task/product syntax.

Validation

• CI=1 uv run pytest -q passed with 59 passed, 2 skipped, 1 warning.
• git diff --check passed with only line-ending warnings from Git on Windows.

Notes

ruff could not be run locally because the ruff executable is not installed in this environment.