Add unit tests for the backend dto.ts chain-model to DTO mapping

[Baskarayelu]

Closes #989

Summary

• Add src/lib/backend/__tests__/dto.test.ts with comprehensive unit tests for mapCommitmentFromChain and mapAttestationFromChain
• Tests cover happy path, optional/missing fields, numeric type conversions, and unknown status/verdict defaults
• 11 test cases total across both mapping functions

Test coverage

• mapCommitmentFromChain: full model mapping, XLM asset defaults, assetIssuer null for XLM, unknown commitmentType fallback, unknown/missing status fallback, all early_exit variants, numeric type coercions
• mapAttestationFromChain: full model mapping, unknown verdict fallback, details field omission, ISO date fallback for undefined observedAt, numeric timestamp conversion

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