Address review: TableRelation validation carve-out, RecordRef nits

- tablerelation: fields with ValidateTableRelation/TestTableRelation = false
  are skipped by codeunit 134926 and may be longer (filter/totaling
  fields); shorter is still a finding. Add Code/Text type rule, BaseApp
  evidence, a ValidateTableRelation = false filter field to the good
  fixture, and the carve-out to the data-modeling cue.
- recordref: add FindLast/Next, relabel SplitLocalTableFilter as filter
  splitting, mark fixtures as test-library-style helpers.

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

microsoft/knowledge/data-modeling/tablerelation-field-length-must-match-related-field.good.al

@@ -50,6 +50,13 @@ table 50102 "Sample Contract"
             else
             if ("Source Type" = const(Region)) "Sample Region"."Code";
         }
+        // Filter field: holds a filter expression, not one code, so it is
+        // deliberately longer and opts out of relation validation.
+        field(5; "Category Filter"; Code[250])
+        {
+            TableRelation = "Sample Category"."Code";
+            ValidateTableRelation = false;
+        }
     }
     keys
     {