refactor: use slices.Contains to simplify code (#16680)

### Summary

There is a [new function](https://pkg.go.dev/slices@go1.21.0#Contains)
added in the go1.21 standard library, which can make the code more
concise and easy to read.

Signed-off-by: weifanglab <weifanglab@outlook.com>

internal/agent/component/llm_test.go

@@ -15,6 +15,7 @@ package component
 import (
 	"context"
 	"errors"
+	"slices"
 	"testing"
 
 	"github.com/cloudwego/eino/schema"
@@ -178,14 +179,7 @@ func TestLLM_Invoke_InvokerError(t *testing.T) {
 
 func TestLLM_Registered(t *testing.T) {
 	names := RegisteredNames()
-	found := false
-	for _, n := range names {
-		if n == "llm" {
-			found = true
-			break
-		}
-	}
-	if !found {
+	if !slices.Contains(names, "llm") {
 		t.Fatalf("LLM not registered; names=%v", names)
 	}
 	// And a factory round-trip.

internal/agent/component/parallel_test.go

@@ -35,6 +35,7 @@ package component
 
 import (
 	"context"
+	"slices"
 	"testing"
 )
 
@@ -45,14 +46,7 @@ import (
 // assert their absence here.
 func TestParallel_Registered(t *testing.T) {
 	names := RegisteredNames()
-	hasParallel := false
-	for _, n := range names {
-		if n == "parallel" {
-			hasParallel = true
-			break
-		}
-	}
-	if !hasParallel {
+	if !slices.Contains(names, "parallel") {
 		t.Errorf("Parallel not registered; RegisteredNames=%v", names)
 	}
 }