Fix EnumFormatter: last enum label missing due to pre-joined string

[boserh]

EnumFormatter was returning one pre-joined multi-line string per enum column (via lines.join("\n")). When this string was passed through LineFormatter, flat_map treated it as a single element — so the full string was kept intact. However the comma/no-comma logic used labels.map.with_index with { } block syntax, which in some cases produced an off-by-one in the last-element detection, dropping the final label entirely.

Fix:

• Return a flat array of individual comment lines (flat_map + each_with_index) consistent with every other formatter in the codebase
• Add AND t.typtype = 'e' to pg_enum_labels query to prevent joining against a non-enum pg_type with the same name
• Update specs to assert on the flat line array