feat(workflow): add BSON writer support for legacy SystemTask (backward compatibility)

[engalar]

Summary

The BSON parser recognizes Workflows$SystemTask (the legacy activity type name for CallMicroflowTask) for backward compatibility with older Mendix projects. However, there is no BSON writer path for it — the executor always generates Workflows$CallMicroflowTask. If a workflow is read from an old project (containing SystemTask), modified, and written back, the activity type is silently upgraded. This may cause confusion when diffing or version-controlling MPR files.

Gap

Layer	Status
BSON Parser	✅ Recognizes Workflows$SystemTask
BSON Writer	❌ Always emits Workflows$CallMicroflowTask
MDL Grammar	❌ No explicit SYSTEM TASK syntax (by design)
Round-trip fidelity	⚠️ Type name silently upgraded on write

Key Files & Lines

BSON Parser — where the type is recognized

• sdk/mpr/parser_workflow.go:219 — "Workflows$SystemTask" mapped to parseSystemTask(r)
• sdk/mpr/parser_workflow.go:313-315 — parseSystemTask() returns *workflows.SystemTask

SDK Type — where SystemTask is defined

• Search sdk/ for SystemTask struct — check if it has a flag to preserve original type name

BSON Writer — where to add the legacy path

• sdk/mpr/writer_workflow.go — search for CallMicroflowTask serialization; add a branch when original type is SystemTask

Options

1. Preserve on round-trip (recommended for backward compat): track the original $Type in the struct and emit it unchanged on write
2. Always upgrade (current behavior): accept the silent upgrade — simpler, but changes the BSON on every round-trip
3. Document as intentional: add a comment in parseSystemTask explaining this is a deliberate upgrade

Notes

• Workflows$SystemTask was the type name used in Mendix < 10.x before it was renamed to CallMicroflowTask
• Projects opened and re-saved in Studio Pro perform the same upgrade, so option 2 may be acceptable
• This issue has low priority unless users report unexpected diffs in version-controlled MPR v2 projects