Add the system role into messages array

[ShivaniKumar1]

Inf-proxy accepts system as a top-level param and also inside the messages array. Anthropic also accepts both: https://platform.claude.com/docs/en/api/go/messages/create & https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages.

Plano seems to only accept system as a top-level param and NOT in the messages array. Otherwise, plano is returning this error:

failed to obtain model route: plano returned status 400: {"error":{"code":"InvalidRequest","message":"Invalid request","details":{"reason":"Failed to parse request: unknown variant `system`, expected `user` or `assistant` at line 1 column 4677"}}}

Changes:

• Clients can now send "role": "system" inside the Anthropic messages array.
• The remaining messages array contains only user/assistant (so it stays valid for Bedrock).