When model.provider is set to custom:<name>, _supports_vision_override()
previously tried only the runtime provider key ('custom') and the raw
config value ('custom:my-proxy'). It did not try the stripped name
('my-proxy'), which is the actual key under providers: in config.yaml.
This caused native image routing to fall back to text mode even when the
user explicitly declared supports_vision: true on the named provider's
model entry.
Fixes#39963
