fix(kosong): match Kimi's standalone "Unsupported image." rejection (#2362)

isImageFormatError missed the production phrasing "Unsupported image.
Please try another one." — the existing pattern requires a url/format/
type suffix, so the deterministic image rejection never triggered the
media-stripped resend, and the session failed on every later request.

Add a standalone-sentence pattern (punctuation- or end-terminated) to
both the kosong and agent-core-v2 classifiers, keeping the deliberate
boundary that count/size phrasings must not match.

Co-authored-by: fengchenchen <fengchenchen@moonshot.ai>