[FXC-5217] updater to add wind tunnel ghost surfaces to pre-25.8 JSONs

[benflexcompute]

Note

Low Risk
Additive, version-gated migration logic that only appends missing ghost entities and is covered by a new test; low chance of impacting non-wind-tunnel simulations.

Overview
Adds a new 25.8.4 updater step that auto-populates private_attribute_asset_cache.project_entity_info.ghost_entities with the full set of WindTunnelGhostSurface entries when none are present, enabling wind-tunnel surfaces to be selectable in older simulation JSONs.

Bumps package/API version from 25.8.3 to 25.8.4, updates reference JSON fixtures accordingly, and adds a unit test covering the new backfill behavior (ensuring existing ghost entities are preserved).

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