test: add bug-test reproducer for change-action items storage list

Adds an MDL script under mdl-examples/bug-tests/ exercising both
create-with-members and change-with-members. After exec, `mx check`
reports 0 errors and the describe → exec → describe cycle is a
fixpoint, confirming the writer emits the constant storage-list
marker `2` for ChangeActionItem lists and the default
`ErrorHandlingType: Rollback` for ChangeAction.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hjothamendix

mdl-examples/bug-tests/373-change-action-items-storage-list.mdl

@@ -0,0 +1,50 @@
+-- ============================================================================
+-- Bug #373: Writer emitted invalid ChangeActionItem list markers
+-- ============================================================================
+--
+-- Symptom (before fix):
+--   The microflow BSON writer serialized `CreateChangeAction.Items` and
+--   `ChangeAction.Items` arrays with the COUNT of member changes as the
+--   first BSON array element. Mendix storage lists use a constant
+--   storage-list MARKER there. For ChangeActionItem lists this marker is
+--   `2`, independent of how many items the list contains. Writing the
+--   item count produced unstable microflow BSON for create/change object
+--   actions and could lead to invalid action payloads when Studio Pro
+--   re-loaded the model.
+--
+--   Additionally, `ChangeAction` was missing its default
+--   `ErrorHandlingType: Rollback` field, so the written shape did not
+--   match the expected action payload.
+--
+-- After fix:
+--   - Writer emits `bson.A{int32(2)}` for both CreateChangeAction.Items
+--     and ChangeAction.Items.
+--   - Writer emits the default `ErrorHandlingType: Rollback` for
+--     ChangeAction.
+--
+-- Usage:
+--   mxcli exec mdl-examples/bug-tests/373-change-action-items-storage-list.mdl -p app.mpr
+--   mxcli -p app.mpr -c "describe microflow BugTest373.MF_CreateAndChange"
+--   `mx check` against the resulting MPR must report 0 errors.
+-- ============================================================================
+
+create module BugTest373;
+
+create entity BugTest373.Order (
+  Name : string(100),
+  Status : string(50)
+);
+/
+
+-- Create-with-members and change-with-members both exercise the
+-- ChangeActionItem storage list. After exec, the MPR must load cleanly
+-- in Studio Pro (`mx check` reports 0 errors).
+create microflow BugTest373.MF_CreateAndChange (
+  $Name: string
+)
+returns BugTest373.Order as $Order
+begin
+  $Order = create BugTest373.Order (Name = $Name);
+  change $Order (Status = 'Processed');
+end;
+/