The configure flow stores auth credentials under `provider: "volcengine"`,
but the coding model uses `volcengine-plan` as its provider. Add a scoped
`normalizeProviderIdForAuth` function used only by `listProfilesForProvider`
so coding-plan variants resolve to their base provider for auth credential
lookup without affecting global provider routing.
Closes#31731
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>