fix: issue #39

[nikhilkumar1612]

Summary

Closes #39.

This PR fixes precision drift in relayer_getFeeData.rate without changing the API shape. The rate field remains a numeric value in the response, but RelayX now computes it using exact scaled integer math and rounds it deterministically to 6 decimal places before serialization.

Problem

relayer_getFeeData was deriving rate from Chainlink-style oracle values using f64 arithmetic. That can introduce binary floating-point precision and rounding inconsistencies, especially when clients use the returned rate for fee calculations.

What Changed

• Replaced the direct f64-based rate calculation in src/methods/fee_data.rs with scaled U256 integer math.
• Added a deterministic half-up rounding rule to 6 decimal places for relayer_getFeeData.rate.
• Kept the response contract unchanged:
  • rate is still returned as a JSON number
  • FeeDataResponse.rate remains f64
• Added unit tests covering:
  • half-up rounding at 6 decimals
  • recurring-decimal behavior
  • invalid/non-positive oracle answers
• Updated docs to document the 6-decimal rounding rule.

Why This Approach

The issue is not that rate is a number on the wire. The issue is that the number was previously derived using floating-point math.

This change preserves backward compatibility for clients while making the returned value deterministic and easier to reason about.

Testing

• cargo test --lib

Notes

• Native token responses are unchanged and still return rate: 1.0.
• Stub mode behavior is unchanged.
• relayer_getQuote is intentionally not changed in this PR; this fix is scoped to relayer_getFeeData / relayer_getExchangeRate.

[ch4r10t33r]

LGTM