[WIP] fix: retain recently written external resources missing from a stale update

[csviri]

An update of the whole resource set of a primary (a poll result or a received
event) might have been created before the reconciler wrote a resource, thus not
containing it yet. Since such updates are handled as the full actual state, the
write was lost from the cache, and the next reconciliation created a duplicate
of an already created resource or repeated an already executed update.

Writes are now marked as unconfirmed and retained for the next update if it
either does not contain the resource at all - the expected case for a create -
or still contains the state that the write replaced. Any other state is treated
as a change made outside of the reconciler and accepted as actual. Marks are
dropped on the first update, so a resource really deleted or changed meanwhile
is not retained indefinitely.

Also guards handleRecentResourceUpdate against a missing cache entry, and
resolves the actual resources from the state resources in the external state
bulk dependent integration test, which is the recommended approach for
resources that take longer to become visible.

[coderabbitai]

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