Milestone 1: Validated Domain

Build the domain in Solution: - static class InvalidExpenseException extends RuntimeException — carries String problem (getter). - record Expense(String id, String category, int cents): - constructor validates: id/category non-blank, cents > 0 — failures throw InvalidExpenseException with a specific problem message (e.g. "blank category", "non-positive cents") - Expense is a hash-contract value type (records already are — note WHY in a comment). This is Module 1 + 6 fused: fail fast, exceptions carry data.

Difficulty: intermediate