A channel post containing an @token that is not a syntactically valid
username (too short, leading digit, etc.) made messages.sendMessage return
500 INTERNAL_SERVER_ERROR: mentionedUserIDsFromMessage turned every
ResolveUsername error into internalErr().
Skip tokens that fail with ErrUsernameInvalid / ErrUsernameNotOccupied
instead, matching real Telegram (the message sends, the client renders the
mention and only fails to open it on tap). Only unexpected storage errors
still abort. Same fix applied to the bot send path.
