Chat creation and chat moves each carried their own copy of the same folder_id validation, resolving the folder and checking ownership and shared write access in slightly different ways. Both now call a single has_folder_write_access helper, which the chat-completions creation path uses as well, so ownership, inherited write grants and nonexistent or malformed ids behave identically everywhere a chat folder_id is set. The owner case also costs one query fewer than before.
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