fix: validate Fairycloak integer responses

[Kewe63]

Fixes #11

Summary

• replace the catch-all BigInt fallback with strict unsigned-integer response validation
• reject malformed, negative, fractional, unsafe-number, and missing required Fairycloak values instead of converting them to 0n
• apply the validation consistently to account counters, all anonymous/non-anonymous fee getters, and prepaid balances
• preserve the existing 0n compatibility default only for absent counters on accounts that do not exist

Why

The previous helper caught every BigInt(...) conversion error and returned 0n. That made malformed relay data indistinguishable from a legitimate zero fee, nonce, transaction ID, or balance, allowing later logic to proceed with false state.

Verification

• regression suite on the previous implementation: 5 tests run, 4 failed with missing expected rejections
• node --test test/relay-integer-parsing.test.mjs — 5 passed
• npm run check — ABI check passed; 70 tests passed
• git diff --check — passed

The regression matrix covers decimal and hex strings, bigint/number values, explicit zero, malformed and whitespace strings, negative/fractional/unsafe numbers, booleans/objects, missing counters on absent versus existing accounts, and malformed prepaid balances.

Compatibility

Valid non-negative integer strings, safe integer numbers, bigints, hexadecimal values, and explicit zero retain their existing results. Only malformed or missing required response fields now fail closed with a field-specific error.