What this does
Fixes #6621.

When several on demand feature views share a source feature view but select different input columns, _get_feature_views_to_use appended the shared source view once per ODFV, each copy carrying only that ODFV's projection. The membership guard if source_fv not in fvs_to_use never matched, because BaseFeatureView.eq compares projections and the freshly-fetched full-projection view never equals an already-stored narrowed copy.

Downstream, _group_feature_refs keys its view index by projection.name_to_use(), so the duplicate entries collide and the last one wins, the other ODFVs' source projections are silently dropped. During get_historical_features this means the losing ODFV's input columns are never fetched, so transforms receive missing inputs (NaN in pandas mode).

The offline retrieval path is masked on 0.62+ by the ref-injection loop from #6140, but the resolution helper still returns duplicate entries with conflicting projections to its other callers (_get_online_request_context, retrieve_online_documents, retrieve_online_documents_v2).

The fix
In _get_feature_views_to_use, index accumulated source views by projection.name_to_use() and, on a repeat hit, merge the projections (union of features, deduped by name) instead of appending a duplicate. A shared source view now resolves to a single entry carrying the union of the required input columns.

The merge helper reassigns projection.features to a fresh list rather than mutating in place, because the projections in play are shallow copies (copy.copy) that share their features list with the registry object.

Tests
Added TestGetFeatureViewsToUseSharedSource in tests/unit/test_utils.py:

• test_shared_source_projections_are_merged two ODFVs (odfv_a on src_fv[["a"]], odfv_b on src_fv[["b"]]) resolve to a single src_fv entry with projection ["a", "b"].
• test_shared_source_ref_order_independent result is the same regardless of ODFV request order.

Both fail on master (assert 2 == 1 the duplicate entries) and pass with this change. ruff check and ruff format are clean.

Related
#6099, #6140