A custom:<name> main provider resolves at runtime to the bare provider id
"custom". In the vision auto-detect chain, the main-provider branch called
resolve_provider_client("custom", ...) WITHOUT explicit_base_url/api_key,
so it returned (None, None) ("no endpoint credentials found") and the whole
chain fell through to OpenRouter/Nous. A user on a custom endpoint with no
aggregator configured then got "No LLM provider configured for task=vision
provider=auto" on every image, even though their main model fully supports
vision.
Recover the live endpoint that set_runtime_main() records each turn
(_RUNTIME_MAIN_BASE_URL/_API_KEY/_API_MODE) and forward it to Step 1, with
a fallback to _resolve_custom_runtime() for non-gateway callers. Mirrors the
existing explicit-base_url branch directly above.
Adds TestResolveVisionCustomProvider covering custom, custom:<name>, and the
no-runtime fallback path.
25aa626cb4