Implement Search() in Infinity in GO (#13645)

### What problem does this PR solve?

Implement Search() in Infinity in GO.

The function can handle the following request. 
"search '曹操' on datasets 'infinity'" 
"search '常胜将军' on datasets 'infinity'"
"search '卓越儒雅' on datasets 'infinity'"
"search '辅佐刘禅北伐中原' on datasets 'infinity'"

The output is exactly the same as  request to python Search()

### Type of change

- [ ] New Feature (non-breaking change which adds functionality)

internal/engine/infinity/client.go

@@ -20,24 +20,66 @@ import (
 	"context"
 	"fmt"
 	"ragflow/internal/server"
+	"strconv"
+	"strings"
+
+	infinity "github.com/infiniflow/infinity-go-sdk"
 )
 
-// Engine Infinity engine implementation
-// Note: Infinity Go SDK is not yet available. This is a placeholder implementation.
+// infinityClient Infinity SDK client wrapper
+type infinityClient struct {
+	conn   *infinity.InfinityConnection
+	dbName string
+}
+
+// NewInfinityClient creates a new Infinity client using the SDK
+func NewInfinityClient(cfg *server.InfinityConfig) (*infinityClient, error) {
+	// Parse URI like "localhost:23817" to get IP and port
+	host := "127.0.0.1"
+	port := 23817
+
+	if cfg.URI != "" {
+		parts := strings.Split(cfg.URI, ":")
+		if len(parts) == 2 {
+			host = parts[0]
+			if p, err := strconv.Atoi(parts[1]); err == nil {
+				port = p
+			}
+		}
+	}
+
+	conn, err := infinity.Connect(infinity.NetworkAddress{IP: host, Port: port})
+	if err != nil {
+		return nil, fmt.Errorf("failed to connect to Infinity: %w", err)
+	}
+
+	return &infinityClient{
+		conn:   conn,
+		dbName: cfg.DBName,
+	}, nil
+}
+
+// Engine Infinity engine implementation using Go SDK
 type infinityEngine struct {
 	config *server.InfinityConfig
+	client *infinityClient
 }
 
 // NewEngine creates an Infinity engine
-// Note: This is a placeholder implementation waiting for official Infinity Go SDK
 func NewEngine(cfg interface{}) (*infinityEngine, error) {
 	infConfig, ok := cfg.(*server.InfinityConfig)
 	if !ok {
 		return nil, fmt.Errorf("invalid infinity config type, expected *config.InfinityConfig")
 	}
 
+	client, err := NewInfinityClient(infConfig)
+	if err != nil {
+		return nil, err
+	}
+
 	engine := &infinityEngine{
 		config: infConfig,
+		client: client,
 	}
 
 	return engine, nil
@@ -48,12 +90,22 @@ func (e *infinityEngine) Type() string {
 	return "infinity"
 }
 
-// Ping health check
+// Ping checks if Infinity is accessible
 func (e *infinityEngine) Ping(ctx context.Context) error {
-	return fmt.Errorf("infinity engine not implemented: waiting for official Go SDK")
-}
-
-// Close closes the connection
-func (e *infinityEngine) Close() error {
+	if e.client == nil || e.client.conn == nil {
+		return fmt.Errorf("Infinity client not initialized")
+	}
+	if !e.client.conn.IsConnected() {
+		return fmt.Errorf("Infinity not connected")
+	}
+	return nil
+}
+
+// Close closes the Infinity connection
+func (e *infinityEngine) Close() error {
+	if e.client != nil && e.client.conn != nil {
+		_, err := e.client.conn.Disconnect()
+		return err
+	}
 	return nil
 }