fix: Allow serving features while a feature view is MATERIALIZING

[alan-gauthier-jt]

What this does

Adds an opt-in registry config flag, serve_features_while_materializing (default false), that lets online serving continue for a feature view while it is in the MATERIALIZING state.

Problem

The lifecycle serving gate introduced in v0.64.0 (#6401) rejects any feature view whose state is not AVAILABLE_ONLINE or STATE_UNSPECIFIED:

Feature view '<name>' is in state 'MATERIALIZING' and cannot serve features.
Only AVAILABLE_ONLINE feature views can serve.

FeatureStore.materialize() transitions a feature view to MATERIALIZING and commits that state to the registry. In the standard pattern — one shared registry, periodic incremental materialization, and a separate feast serve process — this means routine materialization interrupts online serving until it completes.

Fixes #6780.

Change

When serve_features_while_materializing: true is set under registry:, MATERIALIZING is added to the set of servable states, so servers keep returning the last-materialized values during materialization. States that never had online data (CREATED, GENERATED) remain gated, and the default (false) preserves current behavior.

The flag follows the existing plumbing pattern of enable_online_feature_view_versioning: a RegistryConfig field, copied onto the registry object in __init__ (proto Registry and SqlRegistry), and read via getattr(registry, ...) in the gate.

registry:
  path: ...
  serve_features_while_materializing: true

Testing

• New unit tests in test_feature_view_state.py covering: MATERIALIZING blocked by default, served when the flag is on, AVAILABLE_ONLINE served regardless, and CREATED still blocked even with the flag on.
• ruff check / ruff format clean; mypy clean; full test_feature_view_state.py suite passes (33/33).

Notes

• Python-only: the Go feature server has no equivalent lifecycle gate, so no Go changes are needed.
• No proto changes.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.10%. Comparing base (1f2584d) to head (a008155).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6789      +/-   ##
==========================================
+ Coverage   47.08%   47.10%   +0.01%     
==========================================
  Files         419      419              
  Lines       51878    51885       +7     
  Branches     7525     7526       +1     
==========================================
+ Hits        24429    24441      +12     
+ Misses      25700    25697       -3     
+ Partials     1749     1747       -2     

Flag	Coverage Δ
go-feature-server	30.58% <ø> (ø)
python-unit	48.41% <100.00%> (+0.01%)	⬆️

Files with missing lines	Coverage Δ
sdk/python/feast/infra/registry/registry.py	62.79% <100.00%> (+0.04%)	⬆️
sdk/python/feast/infra/registry/sql.py	58.27% <100.00%> (+0.05%)	⬆️
sdk/python/feast/repo_config.py	79.13% <100.00%> (+0.10%)	⬆️
sdk/python/feast/utils.py	78.65% <100.00%> (+0.77%)	⬆️

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