feat(mpeinit): dump session_info to the log when debug is on

Guard on the table existing, since a service called directly, or by a
client that predates this change, will not send one.

sas/sasjs/macros/mpeinit.sas

@@ -120,4 +120,25 @@ run;
   %let sasjs_mdebug=1;
 %end;
 
+/**
+  * Dump the session context when debug is on. The client sends a single-row
+  * `session_info` table with every service call (see SasService.request) -
+  * this shows exactly what arrived, which is the quickest way to diagnose a
+  * hook that is not behaving as expected. Guarded on the table existing,
+  * since a service called directly, or by an older client, will not have it.
+  *
+  * The values are client supplied, so they are hints only - never treat them
+  * as a security boundary.
+  */
+%if "&sasjs_mdebug"="1" %then %do;
+  %if %sysfunc(exist(work.session_info)) %then %do;
+    data _null_;
+      set work.session_info;
+      putlog "NOTE: &sysmacroname: work.session_info:";
+      putlog (_all_)(=);
+    run;
+  %end;
+  %else %put NOTE: &sysmacroname: no work.session_info on this request;
+%end;
+
 %mend mpeinit;