MDEV-41099 HANDLER READ missing column privilege check

HANDLER OPEN accepted column-level SELECT grants as table access, but
HANDLER READ returns all columns and never checked column privileges.
Reopen of a handler table (after FLUSH/ALTER) did no checks at all.

Require SELECT on every column (as for SELECT *) when the user has no
table-level SELECT, and redo the privilege checks on reopen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sql/sql_parse.cc

@@ -5808,6 +5808,7 @@ mysql_execute_command(THD *thd, bool is_called_from_prepared_stmt)
       There is no need to check for table permissions here, because
       if a user has no permissions to read a table, he won't be
       able to open it (with SQLCOM_HA_OPEN) in the first place.
+      On implicit reopen mysql_ha_open() checks privileges again.
     */
     unit->set_limit(select_lex);