Sync dotnet code with upstream Python changes (bb9afeb8..bfd42c1e)

[Copilot]

Description

Ports Python-side changes from upstream microsoft/graphrag (commits 64c55522 through bb9afeb8) into the dotnet codebase.

Related Issues

Tracks upstream commits: bfd42c1e (phantom entity filtering), bf1034a7/97045b55 (row transformers), 47fae953 (per-workflow stats), ba9169a2 (finalize graph schema).

Proposed Changes

• GraphUtils.FilterOrphanRelationships — Mirrors Python's filter_orphan_relationships(). Drops relationships whose source/target references a non-existent entity title, preventing broken graph edges from LLM hallucinations.

• RowTransformers (new) — C# port of row_transformers.py. Static helpers for row-level type coercion during streaming table reads: SafeInt, SafeFloat, CoerceList, plus per-model transform methods (TransformEntityRow, TransformRelationshipRow, etc.).

• PipelineRunner.DumpStatsAsync — Writes stats.json to output storage after each workflow completes, matching Python's _dump_stats_json() behavior for monitoring/observability.

• Entity.Frequency / Entity.Degree — New properties matching Python's NODE_FREQUENCY and NODE_DEGREE schema columns used by finalize_entities.

• Relationship.CombinedDegree — New property matching Python's EDGE_DEGREE schema column used by finalize_relationships.

• FromDictionary updates — Both Entity and Relationship deserialization now handles the new fields.

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

Additional Notes

316 tests pass (280 existing + 36 new). Zero build warnings, zero CodeQL alerts. Workflow stub implementations remain unchanged — these changes add the data model and utility infrastructure that the workflow implementations will consume once ported.

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