BERLIN, June 25 (Reuters) - A far-right Alternative for Germany (AfD) candidate won a vote on Sunday to become a district leader in Europe's biggest economy for the first time, a breakthrough for the party which has hit record highs in national polls. The 10-year old AfD, with which Germany's mainstream parties officially refuse to cooperate due to its radical views, won a run-off vote in the Sonneberg district in the eastern state of Thuringia with its candidate garnering 52.8% of the vote. It is the latest success for the party which is riding a wave of popular discontent with...