fix(loader): abort when a post edit hook routes to an unregistered table

A POST_EDIT_HOOK may re-point a changeset at a different table - that is what
lets an empty mirror stand in for a real one. Nothing checked the result, so a
hook pointing at a table with no MPE_TABLES row produced a changeset that could
not be reviewed: mpe_checkrestore resolves the audit table from that row, and
with none found its `select count(*) into: chk from &audtab` collapses to
`from where ...` - ERROR 22-322, syscc=1012, and the approval screen aborts
with a syntax error rather than an explanation.

Validate the target immediately after the hook, while the submitter is still
watching, and fail with a message naming the table and the reason. The target
table must be registered in MPE_TABLES; a hook may only route to a registered
table.

Verified on the test estate against the deployed stagedata service:
- target unregistered -> job cancels with
  'ERROR: target table DCHOOK.ORDERS is not registered in VIYA0846.mpe_tables -
  a post edit hook may only route a changeset to a registered table'
  (MPE_LOADFAIL STATUS 'FAILED - TARGET NOT REGISTERED')
- target registered -> the same submit returns STATUS SUCCESS

sas/sasjs/macros/mpe_loader.sas

@@ -462,6 +462,33 @@ run;
   %return;
 %end;
 
+/* The post edit hook may have re-pointed the changeset at a different table -
+  * that is what lets an empty mirror stand in for a real one.  The target must
+  * itself be registered in MPE_TABLES: the approval screen resolves the table's
+  * audit settings from that row, and a changeset whose base table has no row
+  * cannot be reviewed.  Fail here, while the submitter is still watching. */
+%local target_chk;
+proc sql noprint;
+select count(*) into: target_chk
+  from &mpelib..mpe_tables
+  where tx_from le &now and &now lt tx_to
+    and upcase(libref)="%upcase(&libref)"
+    and upcase(dsn)="%upcase(&ds)";
+%if &target_chk=0 %then %do;
+  %let msg=%str(ERR)OR: target table &libref..&ds is not registered in%trim(
+    ) &mpelib..mpe_tables - a post edit hook may only route a changeset to a%trim(
+    ) registered table;
+  %mpe_loadfail(
+      status=FAILED - TARGET NOT REGISTERED
+    ,now=&now
+    ,mperef=&mperef
+    ,reason_txt=%quote(&msg)
+    ,dc_dttmtfmt=&dc_dttmtfmt.
+  )
+  %mp_abort(msg=&msg,mac=mpe_loader.sas);
+  %return;
+%end;
+
 
 /**
   * send to approve process