What is the ruling of the Sharia on buying agricultural lands and converting them into building plots for residential or commercial purposes, given the existence of laws that prohibit such conversion and the unavailability of building plots within the city?

There is nothing in the Sharia (definition) that prohibits building on agricultural land or selling it for construction, as long as it does not result in harm to people, based on the hadith (definition): "There should be no harm nor reciprocating harm." The state may prohibit building on agricultural land through laws if it sees a benefit in doing so, and people must abide by these laws if they are based on a genuine interest.