[clincoln8]

Goal

Ensure the global root StatVarGroup (dc/g/Root, "Data Commons Variables") is always emitted by the preprocessor into _global triples so that manually defined StatVarGroup hierarchies in MCF have a valid root Node row in Spanner without requiring database seeding.

Key Aspects of the Approach

• Unconditionally call self._ensure_root_group() in Nodes.__init__() (simple/stats/nodes.py).
• Keep c/g/Root ("Custom Variables") gated behind if config.generate_hierarchy(): self.group("") (or explicit nodes.group("") calls) so unused custom root groups are not created for manually defined StatVarGroup hierarchies.

Implementation Details

• Previously, self._ensure_root_group() (introduced in [DCP Init DB] Add the dc/g/Root node in preprocessor, exactly like c/g/Root. Allows us to remove Seeding entirely #780) was only called inside Nodes._default_custom_group() and Nodes.group(), which are not reached in standard dcpbridge imports using variablePerRow CSVs (VariablePerRowImporter) and manually defined StatVarGroup nodes in MCF (McfImporter) when groupStatVarsByProperty is not enabled. As a result, dc/g/Root was absent from the Spanner Node table, causing Mixer's BulkVariableGroupInfo (FilterNodesByTypes) to reject dc/g/Root and return an empty StatVar Explorer hierarchy.
• Updated test_root_stat_var_group_created in simple/tests/stats/nodes_test.py to verify that dc/g/Root is initialized immediately on Nodes(CONFIG) construction while c/g/Root is not created unless requested.
• Updated expected golden triples/JSON-LD shards in entities_importer and runner tests to include dc/g/Root.
• Pinned --python 3.12 on uv run pytest for ingestion-helper in run_test.sh so uv does not resolve requires-python = ">=3.12" to CPython 3.15 in Cloud Build and time out building wheels from source.

Edge Cases, Errors & Logging

• _ensure_root_group() is idempotent (if sc.ROOT_SVG_ID not in self.groups:), so subsequent calls to nodes.group() or _default_custom_group() reuse the existing dc/g/Root entry without duplication.

Testing & E2E Verification

• Unit/Integration Tests: Ran the full simple test suite (pytest tests/ kg_util/, 127 passed) and verified yapf/isort formatting.

[codacy-production]

Up to standards ✅

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[gemini-code-assist]

Code Review

This pull request ensures that the global root StatVarGroup (dc/g/Root) is always created by calling _ensure_root_group() during the initialization of the Nodes class. This guarantees that manually defined StatVarGroup hierarchies in MCF have a root Node in Spanner. The corresponding tests and expected test data files (CSV and JSON-LD) have been updated to reflect the presence of this root group. There are no review comments to address, and the changes look solid.