Make the test suite deterministic on Windows and proxied environments

[abhinavkr26104]

Description

The unit suite is not portable to a supported Windows checkout. On Windows with normal Git CRLF conversion and common proxy environment variables, four otherwise-local tests fail.

Reproduction

On Windows/Python 3.12 after installing the dev dependencies:

python -m pytest -q -n 0 -k "not aiohttp"

Observed failures:

1. tests/test_transform.py:424 (sync and async): tests/sample_file.txt is checked out with CRLF, producing SGVsbG8sIHdvcmxkIQ0K; the assertion hard-codes LF bytes (SGVsbG8sIHdvcmxkIQo=).
2. tests/test_client.py:996 and tests/test_client.py:1936: the proxy tests expect one mount after setting HTTP_PROXY, but inherited NO_PROXY/proxy environment state can cause httpx to produce zero matching mounts.

Result from this audit: 4 failed, 1072 passed, 2 skipped after excluding the separately parameterized aiohttp cases. The remaining suite passes.

Expected behavior

Tests should pass on every OS advertised in pyproject.toml (Operating System :: Microsoft :: Windows) and should isolate themselves from host proxy variables and line-ending conversion.

Suggested fixes:

• Make sample_file.txt binary-stable via .gitattributes, or derive the expected base64 from the file bytes rather than a hard-coded newline convention.
• Clear all relevant proxy/no-proxy environment variables in the proxy tests before setting the values under test.
• Add a Windows CI job, at least for the core unit suite.

Why it matters

Windows contributors currently cannot get a clean test run from an unmodified checkout, and the Linux-only CI matrix cannot catch these regressions.