I do not claim ADHD or mention it unless something specifically like this comes up.
I believe that this is where your case ends. Washington is, for the most part, an at-will state. They can fire or choose not to hire you based on anything that isn't a violation of a protected class' rights.
As you did not tell them about your ADHD when being interviewed, they likely did not violate any laws by choosing not to employ you. All they knew was that you tested positive for certain illicit substances over your claims that you might test positive due to prescription drugs.
Had you made it clear why you might've tested positive (due to ADHD), then they might be in violation of ADA. They've likely already filled the position you interviewed for in good faith ("Weird person claims to not be on drugs, but that test might show a positive. Won't tell us why beforehand. Just... don't bother, next!"). If you were an employer and someone acted what could be interpreted as shiftily ("Hey, man, my drug test might turn up positive. If it does, please let me explain myself. I won't beforehand, though, because I don't wanna!") and are interviewing, saying, a dozen people, would you bother giving the shifty person even a second glance or just go straight for the ones who had good interviews and a clean drug screening?
feels like a violation of privacy or Medical laws. There's no reason they should know or need to know what or why I'm on medication, especially considering it has little to no impact on job performance. Of course they need to know why and what drugs you're on. Otherwise, they have only your word. "Hey, man, I'm totally not doing any drugs. It's just prescription meds. I refuse to tell you what they are or what it's for, though." - Let's say that take you at your word. Then they'd have absolutely no way of telling whether or not you're just using said prescription drugs or eventually branch out into illicit ones. After all, your test will almost always turn out positive for those specific substances.
If you wish for this information to remain private, then apply for a job for which drug testing is not mandatory.
I've held jobs with significantly higher requirements and potential for devastating impact if non-compliance, malicious intent or incompetence occurred while working. Then your former employers did wrong in not routinely drug testing you, not this prospective employer. "He did it too!" is not a valid defense or, in this case, accusation.