docs(python): align from_env secret messaging

[joshrotenberg]

Summary

• update the Python CloudClient.from_env() error message so it names every supported secret env var
• document the legacy key and secret aliases accepted by the Python binding
• strengthen the Python test to assert the alias names are present in the error message

Testing

• cargo test
• pytest python/tests/test_client.py (fails in local env: missing built redis_cloud.redis_cloud extension module during test collection)

Closes #58

[joshrotenberg]

Closing — this branch's Cargo.lock changes would downgrade aws-lc-sys 0.40 → 0.37 and rustls-webpki 0.103.13 → 0.103.9, re-introducing the RUSTSEC-2026-{0044,0045,0049,0098,0099,0104} advisories that #83 closed. The README + Python alias-error wording is small and worth keeping; it can be re-cut against current main when the Python publish (#48) and binding scope (#66) work resumes.