update style guide

STYLEGUIDE.md

@@ -1,11 +1,36 @@
-# Style Guide
+# Style Guide <!-- omit in toc -->
 
-The SAP Cloud SDK for AI follows the style guide from the SAP Cloud SDK for JavaScript.
+The SAP Cloud SDK for AI follows the style guide from the SAP Cloud SDK for JavaScript:
 
-## Code
+- [**SAP Cloud SDK code style guide**](https://github.com/SAP/cloud-sdk-js/blob/main/STYLEGUIDE.md)
+- [**SAP Cloud SDK documentation style guide**](https://github.com/SAP/cloud-sdk/blob/main/STYLEGUIDE.md)
 
-[SAP Cloud SDK code style guide](https://github.com/SAP/cloud-sdk-js/blob/main/STYLEGUIDE.md)
+#### Table of Contents
 
-## Documentation
+- [Code Style Guide Rule Overrides](#code-style-guide-rule-overrides)
+ - [Tests](#tests)
+ - [Use .test in test file names](#use-test-in-test-file-names)
 
-[SAP Cloud SDK documentation style guide](https://github.com/SAP/cloud-sdk/blob/main/STYLEGUIDE.md)
+## Code Style Guide Rule Overrides
+
+The rules below differ from the SAP Cloud SDK for JavaScript.
+
+### Tests
+
+#### Use .test in test file names
+
+Use `.test` not `.spec` to differentiate test files.
+
+❌ Examples of **incorrect** code:
+
+```ts
+/* Don't use .spec.ts */
+test - file.spec.ts;
+```
+
+✅ Examples of **correct** code:
+
+```ts
+/* Use .test.ts */
+test - file.test.ts;
+```