MDEV-25973: fixed the test plugins.test_sql_service

Set the data member THD::m_prepare_observer temporary to nullptr
to avoid running a handler when a table version in TABLE_LIST
differs from a value of version in TABLE_SHARE. It happens
for any table when the method Ed_connection::execute_direct()
is called from execute_sql_command() since the data members
TABLE_LIST::m_table_ref_type and TABLES_LIST::m_table_ref_version
have zero values on opening a table but corresponding members
of TABLE_SHARE doesn't have. If the function execute_sql_command()
is called on handling a Prepared statement it results in
issuing the error ER_NEED_REPREPARE that is not issued in case the
statement is run in regular way. So, to make fix the issue
reset the data member THD::m_prepare_observer before running
Ed_connection::execute_direct() and restoring it on return.

sql/sql_prepare.cc

@@ -6093,6 +6093,7 @@ extern "C" int execute_sql_command(const char *command,
   THD *new_thd= 0;
   int result;
   my_bool qc_save= 0;
+  Reprepare_observer *save_reprepare_observer= nullptr;
 
   if (!thd)
   {
@@ -6113,6 +6114,8 @@ extern "C" int execute_sql_command(const char *command,
 
     qc_save= thd->query_cache_is_applicable;
     thd->query_cache_is_applicable= 0;
+    save_reprepare_observer= thd->m_reprepare_observer;
+    thd->m_reprepare_observer= nullptr;
   }
   sql_text.str= (char *) command;
   sql_text.length= strlen(command);
@@ -6150,7 +6153,10 @@ extern "C" int execute_sql_command(const char *command,
   if (new_thd)
     delete new_thd;
   else
+  {
     thd->query_cache_is_applicable= qc_save;
+    thd->m_reprepare_observer= save_reprepare_observer;
+  }
 
   *hosts= 0;
   return result;