[8.17] Adjust unpromotable shard refresh request validation to allow RefreshResult.NO_REFRESH

[tlrx]

When a primary shard uses the read-only engine, it always returns a RefreshResult.NO_REFRESH for refreshes. Since #93600 we added an extra roundtrip to hook unpromotable shard refresh logic. This hook is always executed, even if there are no unpromotable shards, but the UnpromotableShardRefreshRequest would fail if the primary shard returns a RefreshResult.NO_REFRESH result.

Fix to be backported to several versions as it's annoying.

Closes #129036
Backport of #129176 for 8.17.8

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