fix(mtmd): jinja typo error in Gemma4ChatHandler when video mtmd

[craftingmod]

Summary

In Gemma4ChatHandler, video multimidal JINJA template uses part instead of item, causes

ValueError: GenericMTMDChatHandler(_replace_media_placeholders): media marker mismatch
- marker_count=0
- media_count=1
- media_marker='<__media__>'
Each media item must render to exactly one MTMD media marker. Check whether the chat template rendered both a media tag and the original URL/data URI, or failed to render a media placeholder.

gemma4 could not find media_marker.

This PR corrects typo error.

AI Dispose

Main problem and typo error was founded by Codex, but I'm manually reviewed typo changes.