LOS ANGELES — The first plaintiff to take major social media companies to trial is expected to take the stand Wednesday to speak about the harms she says the tech platforms inflicted on her mental health as a child. The plaintiff, who is now 20 and is identified in court by her initials, K.G.M., is at the center of a bellwether case in Los Angeles County Superior Court that could set a legal precedent about whether social media platforms are responsible for causing mental health issues in children. The trial is the first in a consolidated group of cases brought against Instagram, YouTube, TikTok and Snap by more than 1,600 plaintiffs, including over 350 families and over 250 school districts. The plaintiffs accuse the tech companies of knowingly designing addictive products harmful to young users’ mental health. K.G.M., who was a minor at the time of the incidents outlined in her lawsuit, claims that her early use of social media led to addiction and worsened her mental health problems.…