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.go

@@ -373,6 +373,23 @@ func ConvertProvidersToModelList(cfg *Config) []ModelConfig {
 				}, true
 			},
 		},
+		{
+			providerNames: []string{"avian"},
+			protocol:      "avian",
+			buildConfig: func(p ProvidersConfig) (ModelConfig, bool) {
+				if p.Avian.APIKey == "" && p.Avian.APIBase == "" {
+					return ModelConfig{}, false
+				}
+				return ModelConfig{
+					ModelName:      "avian",
+					Model:          "avian/deepseek/deepseek-v3.2",
+					APIKey:         p.Avian.APIKey,
+					APIBase:        p.Avian.APIBase,
+					Proxy:          p.Avian.Proxy,
+					RequestTimeout: p.Avian.RequestTimeout,
+				}, true
+			},
+		},
 	}
 
 	// Process each provider migration