feat(workflow): preserve Multi-User Task distinction in MDL and BSON writer

[engalar]

Summary

Workflows$MultiUserTaskActivity is recognized by the BSON parser but silently mapped to SingleUserTaskActivity. The writer always generates SingleUserTaskActivity, losing the business distinction between single-user and multi-user task assignment.

Gap

Layer	Status
BSON Parser	⚠️ Recognized but merged into UserTask
BSON Writer	❌ Always emits SingleUserTaskActivity
MDL Grammar	❌ No MULTI USER TASK syntax
MDL Executor	❌ No handler

Key Files & Lines

BSON Parser — where the type is silently collapsed

• sdk/mpr/parser_workflow.go:192 — "Workflows$MultiUserTaskActivity" mapped to parseUserTask(r), losing the type distinction

BSON Writer — where single-user type is always emitted

• sdk/mpr/writer_workflow.go — search for Workflows$SingleUserTaskActivity; no branch for multi-user

SDK Type — where to add the struct

• sdk/ or generated/ — add MultiUserTaskActivity struct (or add IsMultiUser bool flag to existing UserTask)

Grammar — where to add syntax

• mdl/grammar/MDLParser.g4:2223-2231 — add workflowMultiUserTaskStmt to workflowActivityStmt
• mdl/grammar/MDLParser.g4:2234 — reference existing workflowUserTaskStmt for keyword diff

Executor — where to add dispatch

• mdl/executor/cmd_workflows_write.go:159-194 — buildWorkflowActivities()

Proposed MDL Syntax

MULTI USER TASK 'ReviewOrders' 'Review Orders'
  PAGE MyModule.ReviewPage
  TARGETING MICROFLOW MyModule.GetReviewers
  OUTCOMES 'Approve' -> { } 'Reject' -> { };

Notes

Mendix distinguishes Single vs Multi-User Tasks in Studio Pro — Multi-User Tasks allow multiple assignees and require all (or a configurable number) to complete before proceeding.