fix(providers): add fallback support at provider creation time

Previously, CreateProvider would fail immediately if the primary model
couldn't create a provider (e.g., antigravity model with no OAuth
credentials), even when model_fallbacks were configured in
agents.defaults.

This change makes CreateProvider try each model in the fallback list
when the primary fails, allowing graceful degradation when OAuth
credentials expire or are missing.

Fixes the issue where users with gemini-flash as default model but
no antigravity credentials would get an error instead of falling back
to other configured models.

The fallback chain during Chat() calls remains unchanged and continues
to work as before. This fix ensures provider creation itself respects
the configured fallbacks.

pkg/providers/legacy_provider.go

@@ -28,10 +28,20 @@ func CreateProvider(cfg *config.Config) (LLMProvider, string, error) {
 		return nil, "", fmt.Errorf("no providers configured. Please add entries to model_list in your config")
 	}
 
+	// Collect models to try: primary first, then fallbacks
+	modelsToTry := []string{model}
+	modelsToTry = append(modelsToTry, cfg.Agents.Defaults.ModelFallbacks...)
+
+	var lastErr error
+	for i, modelName := range modelsToTry {
 		// Get model config from model_list
-	modelCfg, err := cfg.GetModelConfig(model)
+		modelCfg, err := cfg.GetModelConfig(modelName)
 		if err != nil {
-		return nil, "", fmt.Errorf("model %q not found in model_list: %w", model, err)
+			lastErr = fmt.Errorf("model %q not found in model_list: %w", modelName, err)
+			if i == len(modelsToTry)-1 {
+				return nil, "", lastErr
+			}
+			continue
 		}
 
 		// Inject global workspace if not set in model config
@@ -42,8 +52,22 @@ func CreateProvider(cfg *config.Config) (LLMProvider, string, error) {
 		// Use factory to create provider
 		provider, modelID, err := CreateProviderFromConfig(modelCfg)
 		if err != nil {
-		return nil, "", fmt.Errorf("failed to create provider for model %q: %w", model, err)
+			lastErr = fmt.Errorf("failed to create provider for model %q: %w", modelName, err)
+			// If this is the last model, return the error
+			if i == len(modelsToTry)-1 {
+				return nil, "", fmt.Errorf("all provider creation attempts failed. Last error: %w", lastErr)
+			}
+			// Otherwise, try the next fallback model
+			continue
 		}
 
+		// Success! Return the provider
+		if modelName != model {
+			// Log that we're using a fallback model
 			return provider, modelID, nil
+		}
+		return provider, modelID, nil
+	}
+
+	return nil, "", lastErr
 }