test: make proxy readiness deterministic on Windows

[RobLe3]

Summary

• use an explicit no-proxy opener for the loopback-only real-proxy test;
• preserve the minimal test environment while supplying required Windows process variables;
• stop the readiness loop when the child exits.

Evidence

• uv run --locked --extra dev pytest -q tests/proxy/test_e2e.py — 1 passed
• uv run --locked --extra dev pytest -q — 1,254 passed, 2 skipped, 9 subtests passed
• root cross-repository CI preflight selected and passed the Python portable-local lane

The AWS observation was a diagnostic rehearsal and earns no artifact or qualification credit. A later clean immutable Windows run remains required.

Closes #131