Replace three duplicate ValidateNonNegative* functions with a single
generic implementation using Go 1.18+ generics.
- Add SignedInteger type constraint for generic support
- Create ValidateNonNegative[T SignedInteger] as unified function
- Depprecate ValidateNonNegativeInt64 and ValidateNonNegativeDuration
- Both deprecated functions now call the generic version
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>