fix: honor skipCompare in hexEqual matcher

[Kewe63]

Summary

• honor the declared SkipCompare contract in the custom hexEqual matcher
• skip hexadecimal validation when the expected value is the sentinel
• add regression coverage through the registered Chai plugin

Fixes #399

Root cause

types.d.ts allows SkipCompare as the hexEqual argument, but supportHexEqual() validated the sentinel object with isHex() before applying any skip behavior. The generic skippable plugin only wraps built-in comparison methods, so it could not intercept the custom matcher.

Changes

• detect SkipCompare before validating hexadecimal operands
• exercise hexEqual(skipCompare) in the existing matcher test suite

Testing

• RED, before the implementation change:
  • npx hardhat test ./tests/helpers/matchers/index.test.ts --no-compile
  • result: 14 passing, 1 failing
  • failure: skipCompare was rejected as an invalid hex string
• GREEN, after the implementation change:
  • npx hardhat test ./tests/helpers/matchers/index.test.ts --no-compile
  • result: 15 passing
• make test-unit-hardhat
  • result: 37 passing
• npx eslint tests/helpers/matchers/Hex.ts tests/helpers/matchers/index.test.ts
  • passed
• npx prettier --check tests/helpers/matchers/Hex.ts tests/helpers/matchers/index.test.ts
  • passed

Risk

Low. The early return applies only to the existing SkipCompare sentinel. Normal equality, inequality, case-insensitive comparison, and invalid-hex validation retain their existing paths and coverage.