Overview

Surface featureFlagContinuousInternodeValidationPerDocument through the
rss::PersistenceProvider interface so consumers query a provider capability instead of the feature flag directly, and enable it for disaggregated (Atlas Infinite) storage.

Background

featureFlagContinuousInternodeValidationPerDocument (src/mongo/db/server_feature_flags.idl,
default false, fcv_gated) gates continuous per-document internode validation for Atlas Infinite nodes (SPM-4740). SERVER-128506 added the optional top-level oplog field h (per-document hash).
The flag is currently defined but not consumed anywhere. PersistenceProvider already exposes storage-backend capabilities (e.g. shouldUseReplicatedRecordIds(),
supportsCrossShardTransactions()), and the disaggregated provider gates several of these on feature flags via .isEnabled().

Scope of Work

• src/mongo/db/rss/persistence_provider.h — add capability method, e.g.
  shouldUseContinuousInternodeValidationPerDocument().
• src/mongo/db/rss/attached_storage/attached_persistence_provider.cpp — return false.
• disaggregated_persistence_provider.cpp — return
  gFeatureFlagContinuousInternodeValidationPerDocument.isEnabled() (mirrors
  supportsCrossShardTransactions()).
• src/mongo/db/rss/stub_persistence_provider.h — default + test setter.
• Route the per-document hash write/validate call sites through the new capability.

Acceptance Criteria

• Disagg nodes perform per-document validation only when the flag is enabled and FCV permits.
• Attached storage is unaffected.
• Unit tests cover the capability across all three providers.

Technical Notes

• Touches the write/oplog-apply hot path (per-document SHA256) — run a sys-perf patch.
• Flag is fcv_gated (v9.0); if it later becomes a hard disagg requirement, add it to
  DisaggregatedPersistenceProvider::getMinimumRequiredFCV().