fix: clean up related resources when primary object is deleted

[ifdotpy]

Summary

Fixes #116 — related resources with origin: kcp are now cleaned up when the primary object is deleted, preventing orphaned finalizers that block workspace deletion.

Problem

When a primary object (e.g. ChatUIInstance) is deleted:

1. handleDeletion() deletes the local copy and removes the primary finalizer
2. handleDeletion() returns requeue=true, so processRelatedResources() is never called
3. On the next reconcile, the primary object is gone — the controller has nothing to reconcile
4. Related resources (e.g. a KCP Secret) retain syncagent.kcp.io/cleanup finalizer forever
5. Workspace deletion is blocked indefinitely

Fix

• Process related resources before primary deletion in syncer.go: when the primary object has a DeletionTimestamp, call processRelatedResources(primaryDeleting=true) before the primary syncer runs. This ensures cleanup happens while the local primary copy still exists (needed for reference resolution).
• Add forceDelete field to objectSyncer: when set, triggers handleDeletion() even when the source object's DeletionTimestamp is nil. Used for origin: kcp related resources during primary deletion.
• Remove early return in handleDeletion() that skipped related resource processing with the comment "For now we do not delete related resources".
• Nil-guard for destSide.object after primary deletion to prevent panics when the local copy has already been removed.

Files changed

File	Change
internal/sync/object_syncer.go	Added forceDelete field; triggers deletion without DeletionTimestamp; removed early return
internal/sync/syncer.go	Process related resources before primary deletion; nil-guard for destSide
internal/sync/syncer_related.go	Added primaryDeleting parameter; set forceDelete=true for origin: kcp

Testing

Tested in our Platform Mesh environment (KCP + Gardener MSP clusters):

1. Created ChatUIInstance with related origin: kcp Secret
2. Deleted ChatUIInstance
3. Verified: Secret finalizer removed, workspace deletion completes cleanly

Discussed the approach with @xrstf in Slack — processing deletions in the opposite order was confirmed as the right approach.

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[kcp-ci-bot]

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• bee9e24 fix: clean up related resources when primary object is deleted (bug: Remove syncagent.kcp.io/cleanup finalizer on related resources in kcp #116)

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