fix: upgrade root TypeScript from 4.9.5 to ~5.6.0

@types/node@22 uses syntax that TypeScript 4.9 cannot parse, causing
CI build failures. Upgrade TypeScript to 5.6 and fix type errors:

- ConstrainedTypes.ts: cast Set iterator value (unknown → string)
- TypeScriptInputProcessor.ts: cast between ts.Program versions
  (project TS vs typescript-json-schema bundled TS)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jonaslagoni