fix(agent): route fallback candidates to correct provider

Look up each fallback candidate's provider configuration via
GetModelConfig and CreateProviderFromConfig instead of reusing the
primary agent's provider. This ensures candidates from different
providers (e.g. Gemini, Groq) are routed correctly during fallback
execution.

pkg/agent/loop.go

@@ -518,7 +518,21 @@ func (al *AgentLoop) runLLMIteration(
 			if len(agent.Candidates) > 1 && al.fallback != nil {
 				fbResult, fbErr := al.fallback.Execute(ctx, agent.Candidates,
 					func(ctx context.Context, provider, model string) (*providers.LLMResponse, error) {
-						return agent.Provider.Chat(ctx, messages, providerToolDefs, model, map[string]any{
+						candProvider := agent.Provider
+						candModelID := model
+
+						// Attempt to dynamically route to the correct provider defined in model_list
+						if modelCfg, err := al.cfg.GetModelConfig(model); err == nil {
+							if modelCfg.Workspace == "" {
+								modelCfg.Workspace = al.cfg.WorkspacePath()
+							}
+							if p, mID, err := providers.CreateProviderFromConfig(modelCfg); err == nil {
+								candProvider = p
+								candModelID = mID
+							}
+						}
+
+						return candProvider.Chat(ctx, messages, providerToolDefs, candModelID, map[string]any{
 							"max_tokens":  agent.MaxTokens,
 							"temperature": agent.Temperature,
 						})