Fix signature verification when using OAuth token authentication

[haris-5hah]

Fix signature verification when using OAuth token authentication

Description

This PR addresses an issue with signature verification when the API is instantiated with an OAuth token instead of a key/secret pair. The current implementation fails because it relies on Api::getSecret() which returns null when using OAuth token authentication.

Changes

• Modified verifyPaymentSignature to accept a secret in the attributes when using OAuth tokens
• Updated verifyWebhookSignature to make the secret parameter optional and handle OAuth token scenarios
• Added validation to ensure a secret is always available for signature verification
• Properly namespaced Exception class references

Testing

Tested signature verification with both standard key/secret auth and OAuth token auth.