When a provider has credentials but all are disabled, return 404 (NOT_FOUND)
instead of 400 (BAD_REQUEST). The combo handler already treats 404 as a
fallbackable error, so it will skip to the next model in the chain.
Previously, the 400 status caused the combo to stop with a hard error,
killing the client (e.g., Claude Code) even though other models in the
combo chain were available.
Also changed log level from error to warn since disabled credentials
are an expected configuration state, not an error.
Fixes#334
037d013af8