Fix 804

[langchain4j]

Context

See #804

Change

• OpenAiStreamingChatModel: in case modelName is not one of the known OpenAI models, do not return TokenUsage in the Response. This is done for cases when OpenAiStreamingChatModel is used to connect to other OpenAI-API-compatible LLM providers like Ollama and Groq. In such cases it is better to not return TokenUsage then returning a wrong one.
• For all OpenAI models, default Tokenizer will now use "gpt-3.5-turbo" model name instead of the one provided by the user in the modelName parameter. This is done to avoid crashing with "Model 'ft:gpt-3.5-turbo:my-org:custom_suffix:id' is unknown to jtokkit" for fine-tuned OpenAI models. It should be safe to use "gpt-3.5-turbo" by default with all current OpenAI models, as they all use the same cl100k_base encoding.

Checklist

Before submitting this PR, please check the following points:

• I have added unit and integration tests for my change
• All unit and integration tests in the module I have added/changed are green
• All unit and integration tests in the core and main modules are green
• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)
• I have added my new module in the BOM (only when a new module is added)