fix(api): require warehouse-backed sync surfaces

[jonathanhaaswriter]

Summary

• remove the remaining API sync fallback to direct Snowflake clients and require the configured warehouse surface instead
• update API handlers to report warehouse not configured for warehouse-backed data endpoints
• rewrite sync/API regression tests to use the generic warehouse test double instead of Snowflake clients

Validation

• go test ./internal/api
• go test -race ./...
• GOTOOLCHAIN=go1.26.1 go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.8.0 run --timeout 5m ./...