Make OpenAIChatGenerator easier to extend #13186

Several integrations inherit from OpenAIChatGenerator.

Currently, the base class calls some functions defined at the module level (e.g. _convert_chat_completion_to_chat_message, _convert_chat_completion_chunk_to_streaming_chunk, ...).

This makes it hard for subclasses to customize specific parts and leads to significant code duplication; in addition, the integration implementing this duplicate logic is also forced to test it, so this also costs unnecessary CI runs.

If they were methods of OpenAIChatGenerator, it would be much easier to override them and avoid duplication.

Some integrations worth investigating: Mistral, OpenRouter, STACKIT.

Since some integrations already import these functions, we cannot easily delete them, so an idea might be just exposing them as (static) methods:

class OpenAIChatGenerator:
    @staticmethod
    def _convert_completion_to_chat_message(completion, choice):
        return _convert_chat_completion_to_chat_message(completion, choice)

The exact design needs investigation, as there could be additional extension points worth introducing.

The goal would be letting integrations customize provider-specific behaviors but reusing more of the parent logic: less duplication and testing of copied code.

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