fix: match no-transform directive case-insensitively

[kilisamemarisaaa]

Summary

• match the Cache-Control: no-transform directive case-insensitively, as required by RFC 9111
• add regression coverage for a mixed-case No-Transform response directive

Root cause

The directive check used a case-sensitive regular expression, so valid spellings such as No-Transform and NO-TRANSFORM did not prevent compression.

Verification

• npm test (57 passing, 1 pending)
• npm run lint
• npm run test-ci (100% statements/functions/lines, 95% branches)
• standalone A/B reproduction: lowercase, mixed-case, and uppercase directives now all omit Content-Encoding

Fixes #284