feat: [OpenAI] An AiCore wrapper for Responses API only

[rpanackal]

Context

AI/ai-sdk-java-backlog#364.

An alternative to #806

The above mentioned PR introduced wrapper for all openai endpoints and failed to scale. So, we have stripped down the scope of the wrapper to be limited to Responses API alone.

The motivation for this decision is because, the OpenAI SDK integration is not decidedly "the way to go" but an approach that enables Responses API until we can support code generation for OAS3.2, the original blocker for OpenAI Responses API.

Feature scope:

• AiCoreOpenAiClient.response(OpenAiModel.GPT_5) returns a wrapper AiCoreResponseService

  • AiCoreResponseService provides model injection if left out in params and throws there is a model mismatch against deployment model

  ResponseService responseClient = AiCoreOpenAiClient.responses(OpenAiModel.GPT_5); 
  val params = ResponseCreateParams.builder()
       .input(input)
       .model(ChatModel.GPT_5)
       .store(false)
       .build();
  var response = responseClient.create(params);

Definition of Done

• Functionality scope stated & covered
• Tests cover the scope above
• Error handling created / updated & covered by the tests above
• Aligned changes with the JavaScript SDK
• Documentation updated
• Release notes updated