Fix/realtime test compatibility serialization and tripupdates s14 s16

[jloria2703]

During the implementation of Sub-issue #16, the additions to the Realtime workflow—particularly the ETA module, synthetic fixtures, and the Bytewax-style builder—introduced new imports and helper functions inside the codebase.
These changes affected the structure and expectations of the testing environment, and as a result, some of the unit tests from Sub-issue 14 began failing. The issues were not related to the core logic itself, but rather to how the updated modules were being imported and referenced inside test_realtime.py.

To resolve this, test_realtime.py was updated to restore full compatibility with both sub-issues.
The file was adjusted to correctly reference the modified Realtime utilities, align imports with the new module structure, and ensure that the serialization tests (Sub-issue 14) and the ETA/Bytewax builder tests (Sub-issue #16) could coexist without interfering with each other.

After these fixes, all tests run successfully, but each sub-issue now requires invoking its dedicated test functions explicitly:

Sub-issue 14 — Feed Serialization Tests

python -m unittest tests.test_realtime.GTFSRealtimeTests.test_trip_update_serialization -v
python -m unittest tests.test_realtime.GTFSRealtimeTests.test_vehicle_position_serialization -v
python -m unittest tests.test_realtime.GTFSRealtimeTests.test_alert_serialization -v
python -m unittest tests.test_realtime.GTFSRealtimeTests.test_feed_validation -v

Sub-issue 16 — ETA Module / Bytewax TripUpdates Builder

python -m unittest tests.test_realtime.GTFSRealtimeTests.test_build_trip_updates_bytewax -v