Replace hardcoded global API URL variables with getter functions that check the R_BASE_URL environment variable, falling back to the static defaults. Introduce a joinpath utility in utils.h to safely concatenate a base URL with a path segment, handling trailing slashes and leading path slashes. Remove conditional compilation blocks for RD and OLLAMA backends, consolidating all API URL logic into the new get_completions_api_url and get_models_api_url functions. Update all call sites in openai.h to use the new getter functions instead of direct global variable access.
