fix: bypass gateway for local model connection tests

[Bortlesboat]

Summary

Fix issue #471 by making local provider connection tests bypass the Rowboat gateway even when the user is signed in.

Today testModelConnection() routes all signed-in tests through getGatewayProvider(). That works for Rowboat-managed hosted models, but it breaks local providers because the gateway doesn't know local model ids like llama3.3:70b or qwen3.5:latest.

This patch keeps the current gateway behavior for hosted providers while always testing these local providers directly:

• ollama
• openai-compatible

Changes

• add shouldUseGatewayForModelTest() in apps/x/packages/core/src/models/models.ts
• update testModelConnection() to bypass the gateway for signed-in local providers
• add a focused regression test in apps/x/packages/core/src/models/models.test.ts

Testing

• pnpm test in apps/x/packages/core
  • verifies signed-in ollama bypasses the gateway
  • verifies signed-in openai-compatible bypasses the gateway
  • verifies hosted providers still use the gateway when signed in

Notes

I kept this intentionally narrow to the failing connection-test path. I did not change onboarding model-list behavior because the local-provider UI already falls back to manual model entry.

Fixes #471.