fix: check SIWE ExpirationTime independently of NotBefore

[oniani1]

Fixes #2453

In web3GrantEthereum, the expiration check for SIWE messages is gated on NotBefore != nil:

if parsedMessage.NotBefore != nil && parsedMessage.ExpirationTime != nil && ...

Per EIP-4361, not-before and expiration-time are independent optional fields. When a SIWE message includes an expirationTime but omits notBefore, the entire expiration check is skipped and the expired message is accepted.

The Solana handler already checks them independently and doesn't have this problem.

The MaximumValidityDuration fallback is a separate, broader window based on IssuedAt -- it doesn't respect the per-message ExpirationTime. So a message that sets a short expiration would still be accepted for the full MaximumValidityDuration.

The fix removes parsedMessage.NotBefore != nil && from the expiration check condition, matching the pattern used in the Solana handler.