This is the extent that the Constitution addresses the foreign policy of the U.S., "2: He shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur; and he shall nominate, and by and with the Advice and Consent of the Senate, shall appoint Ambassadors,"
Everything else that you presume to know about what the Constitution says about foreign policy, is a matter of interpreting that phrase. In fact, the phrase "foreign policy," appears zero times in the Constitution.
The president has the power that Congress allows him to have. Otherwise he would not have to get "two thirds," of the Senate to approve the products of the acts involving foreign policy that the founders anticipated he would produce.
Don't suggest that Trump was "rooting out corruption," in acting illegally and in a corrrupt manner by withholding bi-partisan congressionally supported aid from Ukraine for personal/political gain.