docs: add comprehensive comments to AWS SDK v3 mock

- Explain why the mock exists (Jest module resolution issues)
- Document ideal solution (upgrade Jest and ts-jest)
- Specify conditions for safe removal
- Provide testing instructions for future developers

Co-Authored-By: hidetaka@digitalcube.jp <hidetaka@digitalcube.jp>

Author: devin-ai-integration[bot]

__tests__/__mocks__/@aws-sdk/client-cloudfront.js

@@ -1,3 +1,38 @@
+/**
+ * AWS SDK v3 CloudFront Client Mock
+ * 
+ * WHY THIS MOCK EXISTS:
+ * This mock was created to resolve Jest module resolution issues when migrating from AWS SDK v2 to v3.
+ * The older Jest version (24.9.0) and ts-jest (24.0.2) cannot properly resolve Node.js built-in modules
+ * like 'node:stream' that AWS SDK v3 uses internally, causing test failures with errors like:
+ * "Cannot find module 'node:stream' from 'createBufferedReadable.js'"
+ * 
+ * WHAT THE IDEAL SOLUTION SHOULD BE:
+ * Instead of using this mock, the testing infrastructure should be updated to properly support
+ * AWS SDK v3's module resolution patterns. This would involve:
+ * 1. Upgrading Jest to a modern version (>=27.x) that supports Node.js built-in module resolution
+ * 2. Upgrading ts-jest to a compatible version (>=27.x)
+ * 3. Updating Node.js version in CI/CD to >=14.x (already done in package.json engines)
+ * 4. Configuring Jest with proper ESM support if needed
+ * 
+ * WHEN THIS FILE CAN BE REMOVED:
+ * This mock can be safely deleted when ALL of the following conditions are met:
+ * 1. Jest is upgraded to version 27.x or higher
+ * 2. ts-jest is upgraded to a compatible version (27.x or higher)
+ * 3. All tests pass without this mock file present
+ * 4. The real AWS SDK v3 client can be imported and used in the test environment
+ * 
+ * To test if this mock is still needed:
+ * 1. Delete this file temporarily
+ * 2. Run `yarn test`
+ * 3. If tests pass, the mock is no longer needed
+ * 4. If tests fail with module resolution errors, the mock is still required
+ * 
+ * TECHNICAL DEBT:
+ * This mock represents technical debt that should be addressed in a future dependency update cycle.
+ * The mock provides basic functionality for testing but doesn't validate actual AWS SDK v3 behavior.
+ */
+
 module.exports = {
   CloudFrontClient: class MockCloudFrontClient {
     constructor(config) {