MDEV-25886 CHECK TABLE crash with DB_MISSING_HISTORY if innodb_read_only

Occasionally, the test innodb.alter_copy would fail in MariaDB 10.6.1,
reporting DB_MISSING_HISTORY during CHECK TABLE. It started to occur during
the development of MDEV-25180, which introduced purge_sys.stop_SYS().
If we delay purge more during DDL operations, then the test would
almost always fail. The reason is that during startup we will restore
a purge view, and CHECK TABLE would still use REPEATABLE READ
even though innodb_read_only is set and other isolation levels
than READ UNCOMMITTED are not guaranteed to work.

ha_innobase::check(): Use READ UNCOMMITTED isolation level if
innodb_read_only is set or innodb_force_recovery exceeds 3.

dict_set_corrupted(): Do not update the persistent data dictionary
if innodb_force_recovery exceeds 3.

storage/innobase/dict/dict0dict.cc

@@ -5425,7 +5425,7 @@ dict_set_corrupted(
 
 	/* If this is read only mode, do not update SYS_INDEXES, just
 	mark it as corrupted in memory */
-	if (srv_read_only_mode) {
+	if (high_level_read_only) {
 		index->type |= DICT_CORRUPT;
 		goto func_exit;
 	}

storage/innobase/handler/ha_innodb.cc

@@ -14796,10 +14796,9 @@ ha_innobase::check(
 
 	/* We must run the index record counts at an isolation level
 	>= READ COMMITTED, because a dirty read can see a wrong number
-	of records in some index; to play safe, we use always
-	REPEATABLE READ here (except when undo logs are unavailable) */
-	m_prebuilt->trx->isolation_level = srv_force_recovery
-		>= SRV_FORCE_NO_UNDO_LOG_SCAN
+	of records in some index; to play safe, we normally use
+	REPEATABLE READ here */
+	m_prebuilt->trx->isolation_level = high_level_read_only
 		? TRX_ISO_READ_UNCOMMITTED
 		: TRX_ISO_REPEATABLE_READ;