[fix](cloud) Align colocate proc output and tablet health in cloud mode (#60944)

[deardeng]

pick from #60944

• Fix incorrect tablet health statistics in cloud mode for SHOW PROC '/cluster_health/tablet_health': avoid reporting UNRECOVERABLE due to local-mode health checks.
• Add cloud fallback for colocation group detail: when backend sequence metadata is empty, derive backend ids from current tablets.
• In cloud mode, show ReplicaAllocation as null in SHOW PROC '/colocation_group/{GroupId}' for consistent output semantics.
• Keep local mode behavior unchanged.

(cherry picked from commit d1e30df)

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

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[hello-stephen]

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[deardeng]

run buildall

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 7.46% (10/134) 🎉
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