docs: fix README Python examples - #184

Summary

• keep the complete Playwright quickstart inside one Python code fence
• import httpx in the timeout example that uses httpx.Timeout
• add regression tests that compile both README snippets

Why

The quickstart fence ended before the run function, making the rendered example incomplete, and the timeout snippet referenced httpx without importing it.

Fixes #176

Testing

• python -m pytest tests/test_readme.py -q (2 passed)
• full suite: 1219 passed, 8 skipped; 4 pre-existing platform/environment failures tracked in Make the test suite deterministic on Windows and proxied environments #181
• full suite excluding only the four Make the test suite deterministic on Windows and proxied environments #181 cases: 1219 passed, 8 skipped
• python -m ruff check .
• python -m ruff format --check tests/test_readme.py
• python -m pyright tests/test_readme.py

Note: the repository-wide Ruff format check still reports two pre-existing example files (examples/playwright_downloads.py and examples/playwright_proxy.py).