FeatureService cannot pin a specific FeatureView/OnDemandFeatureView version #6717

Problem

A FeatureService exists to freeze the exact feature definitions a model was
trained and served on. Today it cannot pin a specific feature view version —
it always resolves to the active (promoted) version. Feast's own docs name this
limitation verbatim on the alpha feature-view-versioning page:

Feature services — Feature services always resolve to the active (promoted)
version. --no-promote versions are not served until promoted.

Building a service from a version-pinned FeatureView silently falls back to the
promoted version:

fv  = FeatureView(name="driver_stats", version="v2", ...)
svc = FeatureService(name="model_v1", features=[fv[["conv_rate"]]])
svc.feature_view_projections[0].version_tag    # -> None   (should be 2)
svc.feature_view_projections[0].name_to_use()  # -> "driver_stats"  (should be "driver_stats@v2")

Root cause

1. FeatureService.__init__ appends the source view's projection without
   translating its version string into projection.version_tag (the field
   name_to_use() checks to render fv@v2).
2. utils._get_feature_views_to_use hard-codes the version as None for the
   FeatureService branch, so retrieval always falls through to the promoted
   snapshot via get_any_feature_view.

While fixing this, a second pre-existing bug surfaced: offline retrieval of a
version-pinned OnDemandFeatureView fails. Offline stores re-fetch ODFVs from the
registry via an unversioned registry.list_on_demand_feature_views, so a pinned
ODFV ref (odfv@v1:feat) raises ValueError: Could not find feature view from reference ... or silently drops the ODFV's features. Every offline backend is
affected (they funnel through OnDemandFeatureView.get_requested_odfvs or
offline_utils.get_feature_view_query_context, plus dask's inline duplicate).

Proposal

• Capture the pinned version into the service's projection, and read it back at
  retrieval time (fixes both online and offline for plain FeatureViews, which
  share _get_feature_views_to_use).
• Add a clean pinning API: allow plain string refs as FeatureService.features
  entries ("driver_stats@v2:trips_today"), resolved against the registry at
  feast apply time — no importing/reconstructing historical view objects.
• Fix offline ODFV resolution with a shared version-aware helper.

Gated by the existing registry.enable_online_feature_view_versioning flag,
which (despite its name) gates both online and offline versioned resolution.