FEATURE: Use the Qiskit Code Assistant OpenAI API-compatible endpoints by default

[cbjuan]

Is your feature request related to a problem? Please describe.
The Qiskit Code Assistant supports a OpenAI API-compatible endpoints https://docs.quantum.ibm.com/guides/qiskit-code-assistant-openai-api. This extension should use them by default replacing the custom existing ones

Describe the solution you'd like
Replace the current custom endpoints from the Qiskit Code Assistant by the OpenAI API-compatible endpoints https://qiskit-code-assistant.quantum.ibm.com/docs#/OpenAI%20Compatibility

Describe alternatives you've considered
NA

Additional context
This extension already supports OpenAI APIs for local inference with Ollama.