[unit: providers] Slice 8: Provider test button + test-gated creation

[jayfalls]

Summary

• TestButton.svelte component on ProviderCard for testing existing providers
• Create flow changed: "Test & Save" — provider is only created if the test passes
• Edit flow unchanged (save immediately)

Create flow

1. Fill form → click "Test & Save"
2. Provider is created, then tested automatically
3. Test passes → dialog closes, provider stays in list
4. Test fails → provider is deleted (rollback), error shown, form stays open

How to validate

1. Click "Add Provider" → fill form → "Test & Save"
2. With valid credentials → dialog closes, provider appears in list
3. With invalid credentials → error shown, form stays open, provider never appears in list
4. Existing providers show Test button on card