Add data handling and test isolation guidance

- add SCM guidance for deriving base quantities through line unit-of-measure validation`n- add security guidance for parameterizing SetFilter with external text`n- add test isolation guidance for resetting per-test state before initialization guards`n- add web-service guidance for JSON null handling and invariant standard format 9`n- route and cover all five rules with paired evaluation fixtures

microsoft/knowledge/web-services/check-json-null-before-converting-values.good.al

@@ -0,0 +1,28 @@
+codeunit 50172 "Contact Payload Reader Good"
+{
+    procedure ApplyPayload(var Contact: Record Contact; Payload: JsonObject)
+    var
+        EmailAddress: Text;
+    begin
+        if TryGetText(Payload, 'email', EmailAddress) then
+            Contact.Validate("E-Mail", CopyStr(EmailAddress, 1, MaxStrLen(Contact."E-Mail")));
+        Contact.Modify(true);
+    end;
+
+    local procedure TryGetText(Payload: JsonObject; PropertyName: Text; var Value: Text): Boolean
+    var
+        Token: JsonToken;
+    begin
+        if not Payload.Get(PropertyName, Token) then
+            exit(false);
+        if not Token.IsValue() then
+            Error(NotAValueErr, PropertyName);
+        if Token.AsValue().IsNull() then
+            exit(false);
+        Value := Token.AsValue().AsText();
+        exit(true);
+    end;
+
+    var
+        NotAValueErr: Label 'The property %1 must contain a single value.', Comment = '%1 = JSON property name';
+}