Summary

• Normalize HTTP Content-Type media types to lowercase before comparing them with the expected export format.
• Add regression tests for mixed-case CSV and NDJSON content types.
• Verify behavior for both synchronous and asynchronous clients.

Testing

• python -m pytest -q — 144 passed
• python -m ruff check proofline/exports.py tests/test_http.py — passed
• python -m ruff format --check proofline/exports.py tests/test_http.py — passed
• git diff --check — passed

Motivation

HTTP media type values are case-insensitive. This change prevents valid export responses from being rejected because their Content-Type header uses different capitalization.