rename test_index.py to test_index_document.py

.cursor/skills/tdd/interface-design.md

@@ -0,0 +1,33 @@
+# Interface Design for Testability
+
+Good interfaces make testing natural:
+
+1. **Accept dependencies, don't create them**
+```python
+# Testable
+def process_order(order, payment_gateway):
+    pass
+
+# Hard to test
+def process_order(order):
+    gateway = StripeGateway()
+
+```
+
+
+2. **Return results, don't produce side effects**
+```python
+# Testable
+def calculate_discount(cart) -> float:
+    return discount
+
+# Hard to test
+def apply_discount(cart) -> None:
+    cart.total -= discount
+
+```
+
+
+3. **Small surface area**
+* Fewer methods = fewer tests needed
+* Fewer params = simpler test setup