Regression test for CVE-2026-43644, mirroring the TestStoreReadHandler_ContentType
test added in #463. Verifies the echoHandler direct-response branch returns
application/octet-stream, X-Content-Type-Options: nosniff, and a restrictive CSP
so an HTML payload cannot be MIME-sniffed and executed.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>