feat: add Avian as a named LLM provider

Add Avian (https://avian.io) as an OpenAI-compatible provider with
API base https://api.avian.io/v1 and AVIAN_API_KEY env var support.

Models: deepseek/deepseek-v3.2, moonshotai/kimi-k2.5, z-ai/glm-5,
minimax/minimax-m2.5. Supports chat completions, streaming, and
function calling.

Changes:
- Add Avian to ProvidersConfig struct, IsEmpty(), HasProvidersConfig()
- Add avian protocol to factory provider and default API base
- Add avian case to legacy provider selection (factory.go)
- Add avian migration rule for old config format
- Add default model entries to ModelList (deepseek-v3.2, kimi-k2.5)
- Add avian to example config
- Update AllProviders test count from 18 to 19

pkg/config/migration_test.go

@@ -160,6 +160,7 @@ func TestConvertProvidersToModelList_AllProviders(t *testing.T) {
 			Antigravity:   ProviderConfig{AuthMethod: "oauth"},
 			Qwen:          ProviderConfig{APIKey: "key17"},
 			Mistral:       ProviderConfig{APIKey: "key18"},
+			Avian:         ProviderConfig{APIKey: "key19"},
 		},
 	}