fix: preserve string-typed invoice dates during deserialization

[miachillgood]

Summary

• stop deserializing string-typed accounting fields into Date objects
• preserve invoice date and dueDate exactly as string values when the model type says string
• add regression coverage for invoice deserialization

Why

Issue #746 reports that paged invoice responses populate Invoice.date and Invoice.dueDate as runtime Date objects even though the SDK model types declare them as string. The root cause is the generic accounting deserializer special-casing /Date(...) string values and converting them to Date even when the target field type is string.

Keeping string-typed fields as strings preserves the declared TypeScript contract and avoids implicit timezone interpretation on date-only values.

Validation

• npm test -- --runInBand
• npm run build

Closes #746