The cleric can't just transform their metal cell door to escape
It seems that your concern is that, should a Forge Domain cleric be imprisoned in a jail cell made of metal, they could use Artisan's Blessing to deconstruct the metal bars by transforming them into some other object. However, this is not possible. Part of the requirement for using Artisan's Blessing is (emphasis mine):
As part of this ritual, you must lay out metal, which can include coins, with a value equal to the creation.
The requirement to "lay out" the metal implies that the metal to be transformed using this feature cannot be part of a structure or other larger object. Even if the cell door is made of metal, the cleric is not capable of laying out this metal in the manner required by the ritual. Similarly, it wouldn't make sense that the cleric could disable a trebuchet made mostly of wood by transforming the metal hinge into something else.
If the cleric could get their cell door off its hinges and lay it down on the floor of their cell, then they could probably transform it, but at that point they have already escaped their cell.