feat: Add online sequence configuration foundation

[patelchaitany]

Summary

• Add the OnlineConfig dataclass with latest and sequence modes, retention limits, write semantics, and validation.
• Persist online configuration through FeatureView protobuf serialization and registry apply/update flows.
• Display online configuration in feature-views describe and document the experimental contract.
• Add focused coverage for validation, protobuf round trips, backward compatibility, registry apply/update/clear behavior, and CLI output.

Scope

This PR establishes the backward-compatible SDK and registry foundation for future sequence features. Online-store append, sequence retrieval, and backend-specific eviction are intentionally deferred to follow-up PRs.

Acceptance criteria

• OnlineConfig dataclass with mode, max_length, max_age, and write_mode
• FeatureView accepts online_config
• Proto serialization and deserialization
• Sequence validation requires append writes and positive max_length
• Existing behavior remains unchanged when online_config is omitted
• feast apply stores, updates, retrieves, and clears OnlineConfig
• feature-views describe displays configuration details

Validation

• 26 focused tests passed
• Ruff checks passed
• Scoped mypy checks passed for all changed Python modules
• Python protobuf generation and local Go protobuf generation validated
• Pre-commit format, lint, template, and secret checks passed

Full repository mypy currently requires the optional types-PyMySQL stub, which is not installed in the local environment.

Parent epic: RHOAIENG-80117

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 79.10448% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.69%. Comparing base (d9ea120) to head (899c4ce).

Files with missing lines	Patch %	Lines
sdk/python/feast/online_config.py	76.92%	8 Missing and 4 partials ⚠️
sdk/python/feast/cli/feature_views.py	66.66%	0 Missing and 1 partial ⚠️
sdk/python/feast/infra/registry/registry.py	75.00%	0 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6848      +/-   ##
==========================================
+ Coverage   47.62%   47.69%   +0.06%     
==========================================
  Files         422      423       +1     
  Lines       52352    52419      +67     
  Branches     7596     7613      +17     
==========================================
+ Hits        24931    24999      +68     
+ Misses      25646    25636      -10     
- Partials     1775     1784       +9     

Flag	Coverage Δ
go-feature-server	30.58% <ø> (ø)
python-unit	49.03% <79.10%> (+0.07%)	⬆️

Files with missing lines	Coverage Δ
sdk/python/feast/__init__.py	97.14% <100.00%> (+0.08%)	⬆️
sdk/python/feast/batch_feature_view.py	77.46% <100.00%> (+0.32%)	⬆️
sdk/python/feast/feature_view.py	87.26% <100.00%> (+0.24%)	⬆️
sdk/python/feast/cli/feature_views.py	33.08% <66.66%> (+6.02%)	⬆️
sdk/python/feast/infra/registry/registry.py	63.54% <75.00%> (+0.79%)	⬆️
sdk/python/feast/online_config.py	76.92% <76.92%> (ø)

... and 2 files with indirect coverage changes

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