Do external distractions, such as a phone ringing or a neighboring worshiper reciting Surat Al-Fatihah aloud, affect the validity of the prayer and necessitate its repetition?

You do not have to repeat the prayer if you lose your devotion, as this does not invalidate the prayer itself, but it does affect its acceptance and reward. If your lack of devotion is caused by the distractions of others, and you are trying to resist them, you are not blameworthy, because Allah does not burden a soul beyond its capacity. People should observe the sanctity of the mosque and prayer by not disturbing those who are praying.