Fix anthropic system messages

[noa-kaplan]

1. Add system role to AnthropicMessage ROLES constant
2. Implement proper system? method that returns true for system role
3. Add system_hash method for converting system messages to API format
4. Update to_hash method to handle system messages correctly
5. Fix support_system_message? to return true (was incorrectly returning false)
6. Update tests to expect correct behavior for system messages

[andreibondarev]

@noa-kaplan Is the role: "system" getting added? I couldn't find any mention of it: https://docs.anthropic.com/en/api/messages