BiomeClient#parseJsonOutput expected a stale --reporter=json shape
(location.path.file, byte-offset span, description), so the shape guard
dropped every diagnostic against Biome 2.x output and lint() returned []
with no warning. Parse the current schema instead: string location.path,
1-indexed location.start/end {line,column}, and message. Drop the now-dead
offsetsToPositions helper and warn when a non-empty diagnostics array has
no recognizable location, so future schema drift surfaces instead of
silently reporting "no lint issues".
Fixes#8694
roboomp