fix: unable to pass Temperature and MaxTokens in agent (#18223)

### Summary

As title

internal/agent/component/agent.go

@@ -82,7 +82,9 @@ type AgentParam struct {
 	SystemPrompt             string
 	UserPrompt               string
 	Thinking                 string
+	MaxTokens                *int
 	TopP                     *float64
+	Temperature              *float64
 	Tools                    []string                  // Agent-visible tool names resolved into Eino BaseTool instances
 	ToolParams               map[string]map[string]any // node-level tool constructor params keyed by tool name
 	SubAgents                []SubAgentTool
@@ -1041,15 +1043,14 @@ func buildAgentChatModel(ctx context.Context, p AgentParam) (*models.EinoChatMod
 	apiKey := p.APIKey
 	cfg := &models.APIConfig{ApiKey: &apiKey}
 	cm := models.NewChatModel(d, &modelID, cfg)
-	// ChatConfig construction is conditional on TopP being set, unlike
-	// the LLM path which always builds a ChatConfig (Temperature/MaxTokens
-	// pass-through). The asymmetry is intentional: AgentParam has no
-	// Temperature/MaxTokens yet, so building a zero-config ChatConfig
-	// would be dead weight. When AgentParam grows Temperature/
-	// MaxTokens, switch to always-build.
+	// Build ChatConfig when a generation parameter or Thinking is set.
 	var chatCfg *models.ChatConfig
-	if p.TopP != nil || p.Thinking != "" {
-		chatCfg = &models.ChatConfig{TopP: p.TopP}
+	if p.TopP != nil || p.Thinking != "" || p.MaxTokens != nil || p.Temperature != nil {
+		chatCfg = &models.ChatConfig{
+			TopP:        p.TopP,
+			MaxTokens:   p.MaxTokens,
+			Temperature: p.Temperature,
+		}
 		switch p.Thinking {
 		case "enabled":
 			t := true
@@ -1370,6 +1371,14 @@ func mergeAgentParam(base AgentParam, inputs map[string]any) AgentParam {
 		f := v
 		p.TopP = &f
 	}
+	if v, ok := intFrom(inputs, "max_tokens"); ok {
+		f := v
+		p.MaxTokens = &f
+	}
+	if v, ok := floatFrom(inputs, "temperature"); ok {
+		f := v
+		p.Temperature = &f
+	}
 	if v, ok := stringFrom(inputs, "thinking"); ok && v != "" && v != "default" {
 		p.Thinking = v
 	}