Add provider selection instructions for test generation

Updated end-to-end testing documentation to include instructions on choosing a provider (Anthropic or OpenAI) when generating tests. Also added a new, empty Python file for app variation login core.

Author: karangattu

components/outputs/ui/app-variation-login-core.py

@@ -189,6 +189,17 @@ pip install "shiny[test,testgen]"
   - Anthropic: `export ANTHROPIC_API_KEY=...`
   - OpenAI: `export OPENAI_API_KEY=...`
 
+
+#### Choosing a provider for generating tests
+
+```bash
+# Anthropic (default)
+shiny add test --app app.py --provider anthropic --model sonnet
+
+# OpenAI
+shiny add test --app app.py --provider openai --model gpt-5
+```
+
 ::: callout-note
 The generated file is a **starting point**, not a full coverage suite. Iterate toward meaningful, stable assertions aligned with your app’s business logic.
 :::