feat(core): Expose previous entity state on CollectionEvent updates

[grolmus]

Summary

Adds a previousEntity to CollectionEvent, populated for updated events, so subscribers can diff a collection against its pre-update state. Builds on and supersedes #4402 by @flobacher (their original commit is preserved).

Root cause / motivation

CollectionEvent only exposed the post-update entity, so a subscriber wanting to react to what changed on a collection update had no access to the previous values.

The original PR (#4402) added previousEntity but loaded it via a bare getEntityOrThrow with no relations, leaving translations (and the other relations) empty — so the previous snapshot was not actually usable for diffing. It also introduced a second collection load alongside the existing active-channel guard.

Change

• collection-event.ts — new previousEntity?: Collection constructor field, documented with @since 3.8.0. The doc notes its relation set is not identical to entity, so reliable diffing is limited to translations, featuredAsset, assets and scalar columns.
• collection.service.ts — update() loads the pre-update collection once, with relations: ['featuredAsset', 'assets', 'channels', 'parent', 'translations']. This single load both enforces the active-channel guard (unchanged semantics — findOneInChannel still filters by channel) and captures the previous snapshot, which is passed to the updated CollectionEvent. The redundant second load is removed. productVariants/children are deliberately excluded (potentially huge).
• Mutation-safety: TypeORM has no persistent identity map and translatableSaver.update builds a brand-new entity for the save, so previousEntity is a detached pre-update snapshot in all update paths (including filter changes).

Test plan

Automated — extends entity-event-update-state.e2e-spec.ts (the suite that already covers post-update state for Promotion/ShippingMethod/Administrator/PaymentMethod events) with a CollectionEvent case: update a collection's name, assert event.type === 'updated', event.entity has the new name, event.previousEntity has the old name, and that a non-translation relation (channels, parent) is loaded on the snapshot (guards against silently dropping a relation).

Verified against a freshly-built core:

✓ entity-event-update-state.e2e-spec.ts (13 tests)   # incl. the new CollectionEvent case
✓ collection.e2e-spec.ts (78 tests)                  # update() path regression

Without the fix, event.previousEntity is undefined and the new assertions fail.

Closes #4402

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[michaelbromley]

Hi Martin,

The PR this comes from is rightly labeled T3: systemic. That means it should not be blindly implemented.

This is a perfect case in fact, because we should not be ad-hoc solving this for a single event. We need to step back and think about the general problem of "how do we convey the specific changes to an entity via the event system?"

[grolmus]

@michaelbromley ah u are right. i didnot notice that it is T3

thanks for your check