Backend: Snowflake Cortex integration (vector search)

[jongalloway]

This issue tracks adding Snowflake Cortex as a backend option for vector search.

Goals

• Implement a Snowflake-backed vector search for NLWebNet abstractions.
• Document account setup, cost control, and access patterns for Snowflake.
• Provide demos using publicly safe patterns (e.g., proxy creds via secrets).

Tasks

• Create NLWebNet.Backend.Snowflake implementing IDataBackend and vector operations using Cortex functions.
• Configuration schema and binding (account, warehouse/db/schema, credentials, dims, metric).
• Service registration extension and sample config entries.
• Integration tests (skipped by default) using environment variables to opt-in.
• Documentation: setup, cost guardrails, troubleshooting.
• Wire into demo sample with feature flag.

Acceptance Criteria

• Snowflake backend can be enabled via config and used alongside other backends.
• Vector insert/search APIs work and pass integration tests (when enabled).
• Sample + docs allow users with Snowflake accounts to run end-to-end.

Link

• Parent: 🗄️ Advanced Backend Support #40 Advanced Backend Support