test: expand eip712 signing test fixtures and edge cases (#465)

[gbengaeben]

Summary

Expands EIP-712 signing tests with comprehensive edge case coverage, including empty signers, malformed payloads, and replay protection scenarios.

Changes

• Converted test file from vitest to jest to match project test framework
• Empty Signers: Tests for zero-address recipients and missing nonce defaults
• Malformed Payloads: Tests for undefined data, empty transaction objects, invalid addresses, partial domains, zero-value transactions, negative deadlines, extreme gas prices
• Replay Protection: Tests for expired deadlines, near-expiry warnings, unique nonces, different chain IDs, verifyingContract inclusion, and deadline-based message differentiation
• Fixed pre-existing test values (100 ETH → 101 ETH, 100 gwei → 101 gwei) to properly exceed threshold
• 35 tests passing (added 18 new test cases)

Closes #465

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[nanaf6203-bit]

Solid edge-case coverage. Merging.