ATLANTA — A federal judge in Arkansas on Tuesday struck down the state’s law forbidding medical treatments for children and teenagers seeking gender transitions, blocking what had been the first in a wave of such measures championed by conservative lawmakers across the country. The case had been closely watched as an important test of whether bans or severe restrictions on transition care for minors, which have since been enacted by 19 other states, could withstand legal challenges being brought by activists and civil liberties groups.