fix: Add ASSISTANT role to ResponseInputItem message input

[caioSalchesttes]

Add ASSISTANT role to ResponseInputItem and ResponseInputMessageItem

Summary

This PR adds support for the assistant role in both ResponseInputItem.Message.Role and ResponseInputMessageItem.Role enums, enabling server-side conversation state management.

Related Issues

This issue is also mentioned in #585.

Changes

• Added ASSISTANT constant to Role companion objects in both files
• Added ASSISTANT to Known and Value enums
• Updated value() and known() methods to handle ASSISTANT case
• Updated documentation to include assistant as a valid role option

Motivation

When managing conversation state on the server side (the service calling the API), adding assistant messages to the conversation history allows:

1. Client-side state control: The server can maintain and control the full conversation history, including assistant responses, without relying solely on OpenAI's provided history.

2. Flexible history management: By including assistant role messages in the input, applications can:

   • Build custom conversation flows
   • Implement their own history persistence
   • Control which messages are included in context
   • Not necessarily use the history provided by OpenAI

3. API compatibility: This change aligns the SDK with the OpenAI API's support for assistant role in message inputs, enabling full utilization of the API's capabilities.

Testing

• ✅ No linter errors
• ✅ All enum cases properly handled
• ✅ Documentation updated

Files Changed

• openai-java-core/src/main/kotlin/com/openai/models/responses/ResponseInputItem.kt
• openai-java-core/src/main/kotlin/com/openai/models/responses/ResponseInputMessageItem.kt