feat(config): add fallback model support for AI configurations

internal/ai/litellm/client.go

@@ -9,16 +9,21 @@ import (
 )
 
 func New(cfg config.AIConfig, httpClient *http.Client, log *slog.Logger) (*compat.Client, error) {
+	fallback := cfg.LiteLLM.FallbackMetadataModel
+	if fallback == "" {
+		fallback = cfg.Metadata.FallbackModel
+	}
 	return compat.New(compat.Config{
-		Name:           "litellm",
-		BaseURL:        cfg.LiteLLM.BaseURL,
-		APIKey:         cfg.LiteLLM.APIKey,
-		EmbeddingModel: cfg.LiteLLM.EmbeddingModel,
-		MetadataModel:  cfg.LiteLLM.MetadataModel,
-		Temperature:    cfg.Metadata.Temperature,
-		Headers:        cfg.LiteLLM.RequestHeaders,
-		HTTPClient:     httpClient,
-		Log:            log,
-		Dimensions:     cfg.Embeddings.Dimensions,
+		Name:                  "litellm",
+		BaseURL:               cfg.LiteLLM.BaseURL,
+		APIKey:                cfg.LiteLLM.APIKey,
+		EmbeddingModel:        cfg.LiteLLM.EmbeddingModel,
+		MetadataModel:         cfg.LiteLLM.MetadataModel,
+		FallbackMetadataModel: fallback,
+		Temperature:           cfg.Metadata.Temperature,
+		Headers:               cfg.LiteLLM.RequestHeaders,
+		HTTPClient:            httpClient,
+		Log:                   log,
+		Dimensions:            cfg.Embeddings.Dimensions,
 	}), nil
 }