Review findings folded: (1) `parse_model_input` resolved the configured
`custom:<name>` ids through its own `load_config()`, so the `user_providers`/
`custom_providers` the three startup callers pass were ignored for this branch
— two config sources for one decision. It now takes `custom_ids` and the route
builds them from its arguments with `custom_provider_slug`, the same identity
`providers:` entries carry everywhere else. (2) The "typo" guard was wrong: it
fired on legitimate bare-custom tags (`custom:qwen3.5:4b`) and its `None` fell
straight back into the default-provider egress the fix exists to prevent. A
bare `custom` route with an unknown id now 404s on the user's own endpoint,
matching `/model`. Docstring lists the `provider:model` form.
b2d2cf4e24