Refactor test setup and improve mock implementations

- Introduced `createTestApp` utility to standardize Express app creation for tests, including JSON parsing, mock logger, and error handling.
- Updated all route tests to utilize `createTestApp`, enhancing consistency and reducing boilerplate code.
- Implemented `mockLogger` for cleaner test output and easier logging mock management.
- Adjusted passport mocks to ensure proper user authentication simulation across tests.
- Enhanced type safety by augmenting Express Request interface with `req.log` and `req.user` properties.
- Removed redundant code and improved readability in various test files.

src/middleware/errorHandler.ts

@@ -14,8 +14,8 @@ export const errorHandler = (err: HttpError, req: Request, res: Response, next:
     return next(err);
   }
 
-  // Use the request-scoped logger if available, otherwise fall back to the global logger.
-  const log = req.log || logger;
+  // The pino-http middleware guarantees that `req.log` will be available.
+  const log = req.log;
 
   // --- 1. Determine Final Status Code and Message ---
   let statusCode = err.status ?? 500;
@@ -55,10 +55,15 @@ export const errorHandler = (err: HttpError, req: Request, res: Response, next:
     log.error({ err, errorId, req: { method: req.method, url: req.originalUrl, headers: req.headers, body: req.body } }, `Unhandled API Error (ID: ${errorId})`);
   } else {
     // For 4xx errors, log at a lower level (e.g., 'warn') to avoid flooding error trackers.
-    // The request-scoped logger already contains the necessary context.
-    // We log the error itself to capture its message and properties.
-    // No need to log the full request for client errors unless desired for debugging.
-    log.warn({ err }, `Client Error: ${statusCode} on ${req.method} ${req.path}`);
+    // We include the validation errors in the log context if they exist.
+    log.warn(
+      {
+        err,
+        validationErrors: errors, // Add validation issues to the log object
+        statusCode,
+      },
+      `Client Error on ${req.method} ${req.path}: ${message}`
+    );
   }
 
   // --- TEST ENVIRONMENT DEBUGGING ---