Adding llm provider to set when is checked to not repeat the same check

[fscheu]

The function llm_api_check inside core\cli\main.py checks if the configured LLMs are reachable.
As some LLM could be repeated in the configuration, there is a set variable to check if the LLM was already checked, but inside the for loop, the code never add the LLM to the set when is checked.
The last line is what I'm adding in this PR

    checked_llms: set[LLMProvider] = set()
    for llm_config in config.all_llms():
        if llm_config.provider in checked_llms:
            continue

        client_class = BaseLLMClient.for_provider(llm_config.provider)
        llm_client = client_class(llm_config, stream_handler=handler, error_handler=handler)
        try:
            resp = await llm_client.api_check()
            if not resp:
                success = False
                log.warning(f"API check for {llm_config.provider.value} failed.")
            else:
                log.info(f"API check for {llm_config.provider.value} succeeded.")
                checked_llms.add(llm_config.provider)