Claude Console API (e.g., GLM accounts) does not accept messages with
role='system' in the messages array, returning 422 error:
"Input should be 'user' or 'assistant'"
This fix automatically transforms system messages by merging them into
the first user message's content, maintaining compatibility with clients
like opencode that send system role messages.
Changes:
- Add _transformSystemMessages() method to merge system content into user messages
- Apply transformation in both relayRequest() and relayStreamRequestWithUsageCapture()
- Only affects claude-console account type, no impact on official API
Fixes issues where opencode users get 422 errors when using Console API accounts.
