Prevents PositionNotFoundException from crashing mlb-campaign when a
player is placed at a position they cannot play (e.g. an outfielder
listed at Catcher in the Google Sheet). Adds early validation in
get_lineups_from_sheets and proper error handling at all read_lineup
call sites so the user gets a clear message and the game is cleaned up.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>