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/server/config.go

@@ -414,7 +414,11 @@ func Init(configPath string) error {
 
 	// Map doc_engine section to DocEngineConfig
 	if globalConfig != nil && globalConfig.DocEngine.Type == "" {
-		// Try to map from doc_engine section
+		// Use DOC_ENGINE env var if set
+		if docEngine != "" {
+			globalConfig.DocEngine.Type = EngineType(docEngine)
+		}
+		// Try to map from doc_engine section (overrides env var if present)
 		if v.IsSet("doc_engine") {
 			docEngineConfig := v.Sub("doc_engine")
 			if docEngineConfig != nil {