Overview

Estimates served from persisted NDV statistics (analyze mode "ndv", SERVER-131760) currently report EstimationSource::Sampling, which keeps downstream ranking behavior identical to the sample-based estimate but misstates the provenance. Introduce a dedicated source value.

Background

Raised in review of SERVER-131760. Metadata was considered and rejected: it is absorbed by Sampling in the source-merge table, but clampZeroEstimate() treats the two differently (Sampling zeros are bumped to kMinCE, Metadata zeros are not), and join selectivity is 1 / NDV. Provenance is meanwhile visible in explain via
queryPlanner.fieldStatsMetadata (SERVER-131761), so nothing is lost today.

Scope of work

• Add the new value to EstimationSource (estimates.h) and extend the source-merge state table in estimates.cpp (its switches end in MONGO_UNREACHABLE_TASSERT defaults). Decide deliberately how it merges with Sampling: merging to Mixed would flip most join estimates to Mixed, since persisted NDVs combine with sample-derived cardinalities.
• Extend clampZeroEstimate() in cardinality_estimator.cpp.
• Extend the query stats ceMethods mapping in get_executor_helpers.cpp and its IDL counterpart.
• Switch the read path (SamplingEstimatorImpl::tryEstimateNDVFromPersistentStats) to the new source and update affected explain/unit tests.