fix(web3): honor SIWE ExpirationTime when NotBefore is absent

[vatsalpatel]

What kind of change does this PR introduce?

Bug fix; Closes #2453.

What is the current behavior?

In web3GrantEthereum (internal/api/web3.go), the SIWE ExpirationTime check is gated on NotBefore != nil:

if parsedMessage.NotBefore != nil && parsedMessage.ExpirationTime != nil && !parsedMessage.ExpirationTime.IsZero() && now.After(*parsedMessage.ExpirationTime) {

Per EIP-4361, not-before and expiration-time are independent optional fields. A SIWE message that sets expirationTime but omits notBefore causes the entire expiration check to be skipped, and an expired message is accepted.

The Solana handler checks the two fields independently and is unaffected.

MaximumValidityDuration does not compensate: it bounds validity based on IssuedAt, not the per-message ExpirationTime. A message with a 5-minute expiry is still accepted for the full MaximumValidityDuration window.

What is the new behavior?

Drop the parsedMessage.NotBefore != nil && clause so ExpirationTime is honored whenever it is set, independent of NotBefore. The adjacent NotBefore check is already independent and is untouched.

Test coverage is filled out across the NotBefore / ExpirationTime matrix so future regressions are caught:

NotBefore	ExpirationTime	Expected	Test
nil	nil	accept	TestHappyPath_MinimalMessage (existing)
nil	nil (too old)	reject	TestValidationRules_IssedTooLongAgo (new case)
nil	past	reject — the regression	TestValidationRules_Expired (new case)
nil	future	accept	TestHappyPath_FullMessage (new case)
future	nil	reject (NotBefore)	TestValidationRules_ValidatedBeforeNotBefore (existing)
past	past	reject (ExpirationTime)	TestValidationRules_Expired (existing)
past	future	accept	TestHappyPath_FullMessage (existing)
future	future	reject (NotBefore fires first)	TestValidationRules_ValidatedBeforeNotBefore (new case)
NotBefore > ExpirationTime	—	parser rejects (ErrNotBeforeAfterExpiration)	internal/utilities/siwe/parser_test.go (existing)

Additional context

• !parsedMessage.ExpirationTime.IsZero() is kept alongside the nil check. The asymmetry with the Solana handler (value type, not pointer) is intrinsic to the SIWE parser types, not something to normalize.
• Signatures for the four new Ethereum test cases were generated offline with a single deterministic throwaway ECDSA key (0x2638aB94…) to keep the diff tight.