fix(tools/mysqlsql): Handle nil panic and connection leak in invoke (#758)

Copy fix from #757. 

The Invoke function had two bugs:

A panic would occur when scanning a row containing a NULL value in a
TEXT or VARCHAR column. The code did not check for nil before attempting
a type assertion on the scanned value.

The *sql.Rows result was not being closed on all code paths, leading to
connection leaks that could exhaust the database connection pool.

This change corrects both issues. A guard clause now checks for nil
values before processing, and rows.Close() is deferred to guarantee the
connection is released.

internal/tools/mysqlsql/mysqlsql.go

@@ -153,6 +153,7 @@ func (t Tool) Invoke(ctx context.Context, params tools.ParamValues) ([]any, erro
 	for i := range rawValues {
 		values[i] = &rawValues[i]
 	}
+	defer results.Close()
 
 	colTypes, err := results.ColumnTypes()
 	if err != nil {
@@ -167,29 +168,26 @@ func (t Tool) Invoke(ctx context.Context, params tools.ParamValues) ([]any, erro
 		}
 		vMap := make(map[string]any)
 		for i, name := range cols {
+			val := rawValues[i]
+			if val == nil {
+				vMap[name] = nil
+				continue
+			}
+
 			// mysql driver return []uint8 type for "TEXT", "VARCHAR", and "NVARCHAR"
 			// we'll need to cast it back to string
 			switch colTypes[i].DatabaseTypeName() {
 			case "TEXT", "VARCHAR", "NVARCHAR":
-				if rawValues[i] != nil {
-					vMap[name] = string(rawValues[i].([]byte))
-				} else {
-					vMap[name] = nil
-				}
+				vMap[name] = string(val.([]byte))
 			default:
-				vMap[name] = rawValues[i]
+				vMap[name] = val
 			}
 		}
 		out = append(out, vMap)
 	}
 
-	err = results.Close()
-	if err != nil {
-		return nil, fmt.Errorf("unable to close rows: %w", err)
-	}
-
 	if err := results.Err(); err != nil {
-		return nil, fmt.Errorf("errors encountered by results.Scan: %w", err)
+		return nil, fmt.Errorf("errors encountered during row iteration: %w", err)
 	}
 
 	return out, nil