feat(auto-test): introduce auto-test design for prompt-based development

The commit introduces a new auto-test design for prompt-based development, enhancing the testing capabilities of the system. This design ensures that test code is automatically inserted or updated based on the presence and syntax of the test code provided by the developer.

docs/development/test-prompting.md

@@ -1,10 +1,18 @@
 ---
 layout: default
-title: Test Prompts
+title: AutoTest Design
 nav_order: 2
 parent: Development
 ---
 
+# AutoTest Design
+
+## Basic Rule
+
+- if test code exists and LLM return with import syntax, AutoDev will replace all code.
+- if test code exists and LLM return with no import syntax, AutoDev will insert test code after the last import statement.
+- if test code does not exist, AutoDev will insert test code.
+
 ## Test Prompts
 
 Write unit test for following Kotlin code.