test: migrate test suite from Jest to Bun test framework

- Convert test files to use Bun's test framework and mocking utilities
- Update import statements and test syntax
- Add comprehensive test utilities and mock implementations
- Create test migration guide documentation
- Implement helper functions for consistent test setup and teardown
- Add type definitions for improved type safety in tests

__tests__/helpers.test.ts

@@ -1,15 +1,9 @@
-import { jest, describe, it, expect } from '@jest/globals';
-
-// Helper function moved from src/helpers.ts
-const formatToolCall = (obj: any, isError: boolean = false) => {
-    return {
-        content: [{ type: "text", text: JSON.stringify(obj, null, 2), isError }],
-    };
-};
+import { describe, expect, test } from "bun:test";
+import { formatToolCall } from "../src/utils/helpers";
 
 describe('helpers', () => {
     describe('formatToolCall', () => {
-        it('should format an object into the correct structure', () => {
+        test('should format an object into the correct structure', () => {
             const testObj = { name: 'test', value: 123 };
             const result = formatToolCall(testObj);
 
@@ -22,7 +16,7 @@ describe('helpers', () => {
             });
         });
 
-        it('should handle error cases correctly', () => {
+        test('should handle error cases correctly', () => {
             const testObj = { error: 'test error' };
             const result = formatToolCall(testObj, true);
 
@@ -35,7 +29,7 @@ describe('helpers', () => {
             });
         });
 
-        it('should handle empty objects', () => {
+        test('should handle empty objects', () => {
             const testObj = {};
             const result = formatToolCall(testObj);
 
@@ -47,5 +41,26 @@ describe('helpers', () => {
                 }]
             });
         });
+
+        test('should handle null and undefined', () => {
+            const nullResult = formatToolCall(null);
+            const undefinedResult = formatToolCall(undefined);
+
+            expect(nullResult).toEqual({
+                content: [{
+                    type: 'text',
+                    text: 'null',
+                    isError: false
+                }]
+            });
+
+            expect(undefinedResult).toEqual({
+                content: [{
+                    type: 'text',
+                    text: 'undefined',
+                    isError: false
+                }]
+            });
+        });
     });
 });