core/config+cli: empty models are bad; prompt model on provider switch

Reject providers with no models into bad_providers and warn with reasons
on chat/providers. When /provider changes and the current model is not
listed, prompt for a new model (keep it when still supported).

tests/test_config/plyngent-valid.toml

@@ -6,6 +6,9 @@ url = ":memory:"
 preset = "openai"
 access_key_or_token = "sk-1145141919810"
 
+[providers.test1.models]
+"gpt-test" = { text = true }
+
 [providers.test2]
 preset = "openai-compatible"
 url = "https://www.example.com/v1"
@@ -18,6 +21,9 @@ access_key_or_token = "sk-1145141919810"
 preset = "anthropic"
 access_key_or_token = "sk-1145141919810"
 
+[providers.test3.models]
+"claude-test" = { text = true }
+
 [providers.foo1]
 preset = "deepseek"
 access_key_or_token = "sk-1145141919810"