Migrate to OpenAI Responses API

[mzattera]

Currently, OpenAiChatService uses Chat Completions API whilst OpenAiAssistant uses Assistants API (which is deprecated).

Move out of Assistants API and use Responses API instead of Chat Completions API where possible.

[mzattera]

After migration, make sure the API exposes any native object so all API-specific parameters can be accessed, if needed.

[mzattera]

After migration, change OpenAiModelServiceTest to also test models using Response APIs.