CI: match Assisted-by/Generated-by case-insensitively

Co-authored-by already used a case-insensitive check; apply the same
to Assisted-by and Generated-by so casing variants are accepted and
malformed trailers are still rejected.

Assisted-by: Cursor: grok-4.5
Signed-off-by: Matej Hrica <mhrica@redhat.com>