Make assign_asset report only an assignment that holds

[nickschuetz]

What

assign_asset could report success for an assignment that never happened:

• It printed "Assigned" without checking the result of SetComponentProperty, then fell back to a SetComponentProperty addressed by entity id over bus.Event. That doesn't match the reflected call, and it also "succeeded".
• It used azlmbr.math.Uuid() without importing azlmbr.math. That dropped it into a two-argument GetAssetIdByPath that does not match the reflected three-argument signature.
• It treated an unknown asset path as found, because the catalog answers an invalid AssetId, not None.
• It passed the asset id as text rather than the AssetId.

Now

1. Resolve the asset with the reflected three-argument GetAssetIdByPath and check is_valid(). An unknown path is asset_not_found, with a hint to pass the product path.
2. Find the component: an unknown type is component_type_not_found, and one not on the entity is component_not_on_entity.
3. Set the property with the AssetId and check the outcome: set_property_failed.
4. Read the property back before reporting success: assign_asset_failed if it did not stick.

Tests

• Surface: each failure and the read-back success.
• Live (full suite 56 of 56, run together with the other pending fixes): assigning the engine's ground-plane mesh to a Mesh component reads back and succeeds, and an unknown asset is asset_not_found.