[fix](cloud) Exclude decommissioning BE from BE selection (#65221)

[deardeng]

pick from #65221
Problem Summary: Cloud decommissioning backends can still be alive before they reach the final decommissioned state. These backends could still be selected by CloudReplica routing, routine load assignment, and Kafka proxy backend selection. This change treats decommissioning backends the same as decommissioned backends in those selection paths, while preserving current master behavior such as direct multi-replica hashing and routine load blacklist stale-backend cleanup.

Cloud backend selection avoids routing new replica, routine load, and Kafka proxy work to decommissioning backends.

(cherry picked from commit a6ab604)

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

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  • Manual test (add detailed scripts or steps below)
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• Behavior changed:

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[hello-stephen]

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[deardeng]

run buildall