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/defaults.go

@@ -306,6 +306,20 @@ func DefaultConfig() *Config {
 				APIKey:    "",
 			},
 
+			// Avian - https://avian.io
+			{
+				ModelName: "deepseek-v3.2",
+				Model:     "avian/deepseek/deepseek-v3.2",
+				APIBase:   "https://api.avian.io/v1",
+				APIKey:    "",
+			},
+			{
+				ModelName: "kimi-k2.5",
+				Model:     "avian/moonshotai/kimi-k2.5",
+				APIBase:   "https://api.avian.io/v1",
+				APIKey:    "",
+			},
+
 			// VLLM (local) - http://localhost:8000
 			{
 				ModelName: "local-model",