test: raise tuigen coverage to 90% and formatter coverage to 99.5%

tuigen (was 75.4%): covers component call child kinds, struct mount
children, let binding and top-level control flow generation, tag
specific element options, ref binding forms, component element mounts,
goexpr lexing error paths, tailwind hex and gradient parsing, and
parser error paths for components and Go declarations.

formatter (was 77.3%): covers else-if chains, let bindings to calls and
expressions, attribute value forms, orphan comment groups, comment edge
cases, and FormatWithResult and import-fixing paths. Format output is
asserted exactly and checked for idempotency.