BuiltinProviders

The providers llm4s-core ships.

This list is the only place core enumerates providers, and it exists because every client still lives in core. Slice 5 of #1126 moves each client into its own module with its own Llm4sProviderModule, and this list shrinks to whatever is left.

Nothing else should grow a provider list: a new provider is a new ProviderDescriptor plus an entry here, and after slice 5 not even that.