feature: add assert_valid_signature utility function

[CaptainLEVI-XXX]

##Fixes [#1223]

Overview

Adding an internal utility function assert_valid_signature to reduce code duplication in signature validation logic.

Changes

// Before
let is_valid_signature_felt = ISRC6Dispatcher { contract_address: voter }
            .is_valid_signature(hash, signature.into());

// Check either 'VALID' or true for backwards compatibility
let is_valid_signature = is_valid_signature_felt == starknet::VALIDATED
            || is_valid_signature_felt == 1;

assert(is_valid_signature, Errors::INVALID_SIGNATURE);

// After
self.assert_valid_signature(account, hash, signature);

• Added new utility function assert_valid_signature to the InternalTrait implementation
• Updated external function to use the new utility function
• Maintains compatibility with both starknet::VALIDATED and legacy 1 values
• Ran full test suite and formatting checks:
  • ✅ snforge test - All tests passing
  • ✅ scarb fmt -w --check - Code formatting verified

[CaptainLEVI-XXX]

hey @ericnordelo , can you please review this PR?

[ericnordelo]

Hey @CaptainLEVI-XXX. Thanks again for taking the time. We will go through it early this week.