feat: node cannot connect to itself #918 (#1412)

### What problem does this PR solve?

feat: node cannot connect to itself #918

### Type of change

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

web/src/pages/flow/constant.tsx

@@ -254,6 +254,9 @@ export const RestrictedUpstreamMap = {
     Operator.Categorize,
     Operator.Relevant,
   ],
+  [Operator.KeywordExtract]: [Operator.Begin],
+  [Operator.Baidu]: [Operator.Begin],
+  [Operator.DuckDuckGo]: [Operator.Begin],
 };
 
 export const NodeMap = {

web/src/pages/flow/hooks.ts

@@ -297,12 +297,16 @@ export const useValidateConnection = () => {
   // restricted lines cannot be connected successfully.
   const isValidConnection = useCallback(
     (connection: Connection) => {
+      // node cannot connect to itself
+      const isSelfConnected = connection.target === connection.source;
+
       // limit the connection between two nodes to only one connection line in one direction
       const hasLine = edges.some(
         (x) => x.source === connection.source && x.target === connection.target,
       );
 
       const ret =
+        !isSelfConnected &&
         !hasLine &&
         RestrictedUpstreamMap[
           getOperatorTypeFromId(connection.source) as Operator