What this PR does / why we need it:

• Replace ORJSONResponse with Pydantic response_model on /get-online-features and /retrieve-online-documents endpoints, leveraging FastAPI's native Rust-based JSON serialization via Pydantic V2 for optimal performance
• Remove orjson as a direct dependency from pyproject.toml (and regenerated lock files)
• Fix Pixi deprecation warning by replacing [project] with [workspace] in infra/scripts/pixi/pixi.toml

FastAPI docs recommend using Pydantic response_model over custom response classes like ORJSONResponse for JSON serialization performance:

• https://fastapi.tiangolo.com/advanced/custom-response/#orjson-or-response-model
• https://fastapi.tiangolo.com/tutorial/response-model/#response-model-parameter

With response_model, FastAPI serializes directly to JSON bytes via Pydantic's Rust-based serializer skipping the jsonable_encoder intermediate step that ORJSONResponse still required. Pydantic V2 uses the same underlying Rust mechanisms as orjson, so this is at least as fast while also providing automatic response validation and OpenAPI schema documentation.

Which issue(s) this PR fixes:

Fixes #6139