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

config/config.example.json

@@ -222,6 +222,10 @@
     "mistral": {
       "api_key": "",
       "api_base": "https://api.mistral.ai/v1"
+    },
+    "avian": {
+      "api_key": "",
+      "api_base": "https://api.avian.io/v1"
     }
   },
   "tools": {