ATLAS-5400: Fix PUT response when blocking from empty blocked list

[sheetalshah1007]

What changes were proposed in this pull request?

Follow-up to ATLAS-5375 (#729).
If the blocked list on a relationship was empty and a client blocked a tag, the graph was updated correctly — but the PUT response could still show the old edge (tag propagated, nothing blocked). GET right after PUT showed the right state.

This patch fixes that PUT response for empty → blocked (first block and block after unblock):

• Write the blocked list with remove-then-set (copy-on-write)
• Read it back with getListProperty, matching how it is written
• Reload the relationship edge before building the PUT body

Entity tag updates still run in the background; no API change.

How was this patch tested?

• Tested the scenario manually on running Atlas server
• Additionally, added a test which checks that blocking from an empty list updates PUT and GET before the background task finishes. Existing unblock tests still pass.
  mvn -pl repository -Dtest=ClassificationPropagationWithTasksTest,ClassificationPropagationTasksTest,ClassificationTaskTest,ClassificationPropagateTaskFactoryTest test