Refactor and enhance incremental render endpoint tests

- Simplified test structure by introducing helper functions to reduce code duplication for creating worker apps and uploading bundles.
- Improved test cases for the `/bundles/:bundleTimestamp/incremental-render/:renderRequestDigest` endpoint, ensuring robust validation of successful renders and error handling for various scenarios.
- Added tests for handling invalid JSON and missing required fields, enhancing coverage for edge cases in the rendering process.
- Updated tests to ensure proper handling of multiple dependency bundles and improved response validation for different payload conditions.

Author: AbanoubGhadban