doc(testing): Expand end-to-end test generation instructions

docs/end-to-end-testing.qmd

@@ -174,8 +174,33 @@ shiny add test
 ```
 
 1. Answer the prompts: It will ask for the path to your app file (e.g., `app.py`) and a name for your test file (e.g., `test_myapp.py`). Remember, the test file name must start with `test_`.
+1. Generated test files usually include simple assertions that check visible UI text and interactive states for Shiny components (for example, slider values or checkbox output). Treat these as a starting point and expand them to cover additional behaviors, edge cases, and integration points as needed.
 
-1. Edit the generated test file: This command creates a basic test file. You'll need to modify it to add your specific test scenarios (like the slider example above).
+Before generating tests, make sure you have:
+
+- An API key or credentials for the provider you intend to use:
+  - Anthropic: `export ANTHROPIC_API_KEY=...`
+  - OpenAI: `export OPENAI_API_KEY=...`
+
+- A working Shiny app file (e.g. `app.py`).
+- Shiny testing dependencies installed. If you haven't installed the extras yet:
+
+```bash
+pip install "shiny[add-test]"
+```
+
+#### Choosing a provider for generating tests
+
+```bash
+# Anthropic (default)
+shiny add test --app app.py
+
+# OpenAI (will use gpt-5)
+shiny add test --app app.py --provider openai
+
+# explicitly use gpt-5-mini model
+shiny add test --app app.py --provider openai --model gpt-5-mini
+```
 
 ### Troubleshooting Common Issues