[fix][broker][branch-3.3] Fix unloadNamespaceBundlesGracefully can be stuck with extensible load manager (#23349)

[heesung-sohn]

(cherry picked from commit e91574a)

I need to export some of the changes in #23301 to resolve conflicts.

fork-test: heesung-sohn#83

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[heesung-sohn]

I added this change in this PR, 3b9e857 to handle ClusterMigrationTest.testClusterMigrationWithReplicationBacklog failure.

Regarding ClusterMigrationTest.testClusterMigrationWithReplicationBacklog failure, the root cause is that after the broker is disabled, the assignment operation couldn't complete, and its lookup timed out. To handle this, we don't allow assignment when the broker is disabled.

However, this test passes in the master branch now without this change.