BPMN document PUT: optionally assert Data in matchesExpected for symmetry with EF MatchesLoadedSnapshot

## Summary

Follow-up from https://github.com/elsa-workflows/elsa-core/pull/8092 (issue #8064).

Architect + Crew Lead waived Greptile residual P1s on tip `2889d26`: BPMN `matchesExpected` scopes to id/version/StringData/Name/Description/IsLatest/ETag. EF `MatchesLoadedSnapshot` already includes `Data` (variables/options/custom props); Memory CAS is under `store.Sync`. No silent overwrite for landed stores.

Optional 3.9 improvement: also assert `Data` (or equivalent) in BPMN `matchesExpected` for **symmetry** with EF — not a correctness fix for current stores.

## Out of scope for 3.8 / already merged

Do not reopen #8092. This is Backlog / 3.9 only.

Start with PR #8092 and the BPMN document PUT path, then compare its matchesExpected scope with EF MatchesLoadedSnapshot and the Memory store.Sync behavior. Confirm whether Data can be included without changing current-store correctness; done means the BPMN comparison optionally asserts Data for 3.9 symmetry without reopening #8092.

Written by the indexing model from the issue text.