Add Comprehensive Tests and Test Infrastructure Improvements for Gateway Server

### 🚀 Feature Description and Motivation

Add comprehensive unit and integration tests for the gateway server component, including request body handling, routing logic, and error scenarios.

### Use Case

Currently, the gateway server’s core logic lacks thorough automated test coverage. This makes it harder to confidently refactor, extend, or debug the gateway’s routing and request handling features. By introducing robust tests, we can ensure reliability, catch regressions early, and facilitate future development.

### Proposed Solution

Implement new unit tests for the gateway server’s request body handling and routing logic.
Add test helpers and mocks to simplify and standardize test setup.
Refactor and improve the e2e test script for better automation and reliability.
Ensure all new and existing tests pass and provide meaningful coverage for critical gateway paths.

Start by locating the gateway server component and its existing unit, integration, and e2e tests. Review request body handling, routing, and error paths, then identify the test helpers, mocks, and e2e script that need improvement. Done means comprehensive coverage for the stated gateway paths and all new and existing tests passing.

Written by the indexing model from the issue text.