fix: restore gzip middleware body passthrough on Zig runtime

[justrach]

Summary

• fixes compat: gzip middleware on Zig runtime drops Content-Encoding on compressed responses #96 on current main
• normalize request headers to lowercase before Python middleware sees them
• sanitize tunneled header lines before appending them to the Zig response path
• preserve raw bytes bodies in the Zig response bridge so gzip-compressed middleware output is not JSON-serialized to null

Verification

python zig/build_turbonet.py --install
uv run --python 3.14t --extra dev --with requests python -m pytest \
  tests/test_verified_audit_items.py::test_verified_gzip_passthrough_round_trip \
  tests/test_middleware_compat.py::test_gzip_middleware_compat \
  tests/test_middleware_compat.py::test_gzip_body_is_actually_compressed \
  -p no:anchorpy -q

Result: 1 passed, 2 xpassed

Scope

• no redis-related changes
• no version/release metadata changes
• narrow follow-up to the partially merged #110 work