[Feature] Allow explicit OpenAI API mode instead of inferring it from the base URL hostname

The MCP server currently chooses its OpenAI client implementation by checking whether llm.providers.openai.api_url contains an official OpenAI hostname.

use_generic_client = is_non_openai_provider(config.providers.openai.api_url)

if use_generic_client:
    return OpenAIGenericClient(...)
return OpenAIClient(...)

This treats every non-api.openai.com endpoint as a generic Chat Completions-compatible provider. That does not hold for OpenAI proxy services. A proxy may use a non-OpenAI domain while supporting the OpenAI Responses API and embeddings, but not Chat Completions.