test: make suite deterministic on Windows - #189

Summary

• clear both proxy-variable casing variants before setting the value under test
• derive the expected base64 payload from the fixture's checked-out bytes
• add a Python 3.12 Windows CI job using the repository's Steady mock server

Why

Windows treats environment variable names case-insensitively, so deleting lowercase https_proxy after setting uppercase HTTPS_PROXY removed the test value. Normal Git line-ending conversion also changes the fixture bytes from LF to CRLF, while the assertion hard-coded only the LF encoding.

Fixes #181

Testing

• focused regression cases: 4 passed
• full Windows suite: 1221 passed, 8 skipped
• python -m ruff check .
• python -m ruff format --check tests/test_client.py tests/test_transform.py
• python -m pyright tests/test_client.py tests/test_transform.py

Known unrelated baselines: repository-wide Ruff format reports two existing example files, and Python 3.12 mypy reports the TypeAliasType issue fixed separately by #186.