[Test] Add MIME-in-message tests for /checkv3

[Test] Add MIME-in-message tests for /checkv3

Verify that messages with their own MIME structure (multipart/alternative,
multipart/mixed with attachments) are preserved intact when wrapped in
the outer form-data envelope. Unit tests confirm inner MIME boundaries
don't confuse the outer parser; functional tests confirm end-to-end
symbol detection (R_PARTS_DIFFER, MIME_HTML_ONLY) works via /checkv3.