Will one who made a covenant with Allah to stop watching pornography be held accountable if pornographic content appeared to him on the internet unintentionally, and he averted his gaze?

You are not held accountable for anything, and what happened does not diminish your covenant. The fundamental principle is the presumption of innocence, and doubt concerning an act that might invalidate a covenant entails no consequence and necessitates no expiation. Indeed, anything shrouded in doubt is nullified in Islamic law and carries no legal ruling.