Merriam Webster (n.d.) defines solutionist as "a solver of problems
especially : one who makes a practice or occupation of solving puzzles."
So I would describe Solutionist Thinking as viewing problems with an enthusiasm and interest that fuels creative thinking which aids you in solving the observed problem.
According to Collins Dictionary (2019)"A challenge to something is a questioning of its truth or value. A challenge to someone is a questioning of their authority." Based on this definition I would describe Challenge Thinking as critiquing the solution you came up with to solve a problem.