Motivation
Across the fleet we need to know, per cluster, whether the block cache is active. The cache is enabled only when both gBlockCacheSizeBytes > 0 and gBlockCacheNumShards > 0 (see initBlockCache() / pali_block_cache.cpp). There is currently no single stat that reports this state, which the fleet-monitoring Grafana panels depend on.

Approach

• Add a boolean (or 0/1) field to BlockCacheServerStatus set at init time from gBlockCacheSizeBytes > 0 && gBlockCacheNumShards > 0.
• Surface it in the blockCache ServerStatus section (pali_block_cache.cpp) so it is captured by FTDC.

Definition of Done

• db.serverStatus().blockCache reports an enabled/disabled indicator, present in FTDC.
• The value is correct for both the enabled and disabled configurations (verified by a unit test in pali_test.cpp).
• This stat is what the "number of clusters with block cache enabled" Grafana panel consumes.