Regardless if Iran is or is not harboring terrorists against Pakistan.. I am sure Iranian intelligentsia can appreciate the restrain being shown by Pakistan so far.
Arab world, esp the current hawkish leadership, would only be too happy to sponsor an old ally, who knows a thing or two about covert wars, against their mortal enemy. That would be a nightmare for Iran. Brigades of illiterate jihadis, with the backing of an experienced intelligence agency, chanting "death to the infidels" blowing up in Tehran. I pray that Iran Gov comes to its senses before this madness starts.
And now that the Taliban finally have to actually run Afghanistan, they also need something new to distract their masses. And Shia Iran is a very juicy target.
Point being, things can go real bad real quick for Iran if it keeps hosting these rats against Pakistan.