fix: remove redundant union of property types in index signature for unknown additionalProperties
When a schema has both `properties` and `additionalProperties: {}` (or `true`),
the generator now emits `[key: string]: unknown` instead of the redundant union
`[key: string]: unknown | string | null | ... | undefined`.
- When additionalProperties is `unknown`, only use it for the index signature type
(since `unknown` already subsumes all types including `undefined`)
- For typed additionalProperties (e.g. `{ type: 'string' }`), keep current behavior
of including named property types to satisfy TypeScript's index constraint
- Update all affected test snapshots
Co-authored-by: mrlubos <12529395+mrlubos@users.noreply.github.com>