Add edge-case tests for Decimal i128 range and ReDoS-safe parsing

[thlpkee20-wq]

Description

src/lib/decimal.ts validates decimal strings against POSITIVE_DECIMAL_REGEX and range-checks against I128_MAX/I128_MIN for Soroban compatibility, but the boundary cases (exactly I128_MAX, overflow by one, 18 vs 19 fractional digits, ReDoS-style inputs) need explicit tests. Expand src/lib/decimal.test.ts to cover them.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Relevant code: src/lib/decimal.ts, src/lib/decimal.test.ts
• Inputs exceeding 18 fractional digits or i128 range must throw a structured AppError

Suggested execution

• Fork the repo and create a branch
• git checkout -b test/decimal-i128-boundaries
• Implement changes
  • Add tests for values at and just beyond I128_MAX
  • Assert rejection of .5, leading-zero, and 19-digit fractions
  • Add a long-repetition input to confirm bounded-time parsing
• Validate security and correctness assumptions

Test and commit

• Run tests
  • npm test
• Cover edge cases
  • Scaling to Stellar 7-decimal precision, zero, max precision, overflow on conversion
• Include test output and notes

Example commit message

test: cover Decimal i128 range and parsing edge cases

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours

[Elsa-tech2026]

@Elsa-tech2026 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

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This issue has been completed by @Elsa-tech2026 as part of the Stellar Wave Program's 5th Wave 🥳

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