refactor(tests): harmonize UI server tests with codebase style

refactor(tests): harmonize UI server tests with codebase style

- Add helper functions to reduce duplication (createHttpServerConfig, createLargePayload)
- Add waitForStreamFlush() utility for async stream tests
- Add GZIP_STREAM_FLUSH_DELAY_MS constant with documentation
- Remove low-value constant assertion tests
- Add edge case tests for compression boundary and context cleanup
- Harmonize test names to 'Verify X' style
- Use Reflect.get() instead of unsafe double cast
- Replace eslint-disable with proper guard pattern