fix(dryrun): resolve Target from label not Config name

[alexandernorth]

Summary

--dry-run=server against a Config whose name differs from its target's name failed with:

Internal error occurred: Target.config.sdcio.dev "<config namespace.name>" not found

The dry-run path in ConfigStoreHandler.prepareConfigAndTarget discarded the
NamespacedName returned by config.GetTargetKey(labels) and looked up the
Target using the Config's own key instead. The same wrong key was then
propagated as the DatastoreName in the TransactionSet RPC.

Non-dry-run applies were unaffected because the API server only persists the
Config and the actual target resolution happens later in the reconcilers,
which correctly read config.sdcio.dev/targetName / targetNamespace from
the labels.

Changes

• Use the label-derived targetKey for the Target lookup in
  prepareConfigAndTarget.
• Drop the unused key types.NamespacedName parameter from
  RunDryRunTransaction; derive DatastoreName from the resolved *Target
  using storebackend.KeyFromNSN(...).String(), matching the convention used
  by every other call site (pkg/reconcilers/rollout/transaction.go,
  apis/config/target_helpers.go, pkg/reconcilers/targetdatastore/reconciler.go,
  etc.). The previous types.NamespacedName.String() ("ns/name") did not match
  the format the datastore is registered under ("ns.name").

[henderiw]

the most consistent code is if you do this

targetKey := storebackend.KeyFromNSN(target.GetNamespacedName())

iso key -> this would change only 1 place in the dryRun File

[severindellsperger]

LGTM. Maybe address my comment.

[alexandernorth]

the most consistent code is if you do this

targetKey := storebackend.KeyFromNSN(target.GetNamespacedName())

iso key -> this would change only 1 place in the dryRun File

Ok, I adjusted the code to match this

[henderiw]

LGTM