Resolve three conflicts from main's #6680 (supplementUnmappedOpenAIModels):
1. gateway_handler.go — keep PR's allowlist refactoring (ModelAllowlistEnabled,
writeAllowlistedModelsList, modelListingSource); main's OpenAI branch in
writeModelsList was auto-merged.
2. group_models_list.go (modify/delete) — delete file per PR intent; relocate
supplementUnmappedOpenAIModels to group_model_allowlist.go where gateway_service
and openai_codex_models_service can still call it.
3. openai_codex_models_service.go — combine main's supplementUnmappedOpenAIModels
wrapping with PR's ModelAllowlistEnabled() guard.
Also fix two auto-merged test files that still referenced the old
GroupModelsListConfig / ModelsListConfig types.