Fix deserializing lists with @SnsNotificationMessage

[frjonsen]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

When deserialization target is a list, get the inner type

💡 Motivation and Context

#1280

💚 How did you test it?

Modifying the existing test to access the actual fields of the deserialized objects

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated reference documentation to reflect the change
• All tests passing
• No breaking changes

[maciejwalkowiak]

Thanks @frjonsen!