What this PR does / why we need it:

This PR making the feast vector store api with open ai search api compatible so.

This are the changes are made.

1. New OpenAI-compatible endpoint

• Added POST /v1/vector_stores/{vector_store_id}/search that matches the OpenAI vector store search API
• vector_store_id just maps to a feature view name
• Takes a query string, embeds it via LiteLLM, calls retrieve_online_documents_v2, and returns results in OpenAI's vector_store.search_results.page format
• LiteLLM embedding config goes in feature_store.yaml under a new embedding_model section (model, api_key, api_base, api_version, dimensions

2. Metadata filtering

• New filter_models.py with two Pydantic models: ComparisonFilter (eq, ne, gt, gte, lt, lte, in, nin) and CompoundFilter (and/or, nestable)
• Threaded filters through the entire retrieval stack down to each online store
• Each store translates filters into its native query language:
  • Elasticsearch: Query DSL clauses (term, range, terms, bool)
  • Milvus: boolean expressions like field == 'value'
  • Postgres: parameterized SQL subqueries with entity_key IN (SELECT ...)
  • SQLite: same approach as Postgres, SQLite syntax

3. Numeric storage fix

• Without this change, all values are stored as text, so '9' > '100' is true in filters
• New enable_openai_compatible_store config flag on every store backend
• When enabled, adds a value_num column that stores int, float, double, and bool values natively alongside the existing `value_text

4. Bug fixes picked up along the way

• SQLite BM25 search was reading raw value instead of value_text
• SQLite's query param renamed to embedding since that's what it actually is
• Added input escaping for Milvus query strings

5. Tests

• 160 lines of unit tests for filter models (valid/invalid operators, value types, nested compounds)
• ~320 lines of integration tests covering filtered vector search, filtered text search, OpenAI response shape, and error cases (no embedding config, nonexistent feature view, empty results)

Which issue(s) this PR fixes:

#5615

Misc