MAX_SHARE_MESSAGE_CHARS only counted text parts, so a forged non-text part
(tool-result/file/data) bypassed the cap and bloated the model input
(token-DoS); convertToModelMessages would also expand a forged tool-result. The
anonymous path runs no tools, so a client non-text part is never legitimate —
reject any message with a non-text part (isTextUIPart) before the size check.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>