Fix GTFS-RT location inheritance from static feeds

[mackenziereading19]

Summary

Fixes GTFS-RT feeds remaining locationless when they are imported after a referenced static GTFS feed already has location metadata.

During catalogue population, once static_reference values are resolved, a GTFS-RT feed with no location metadata of its own now inherits the locations already attached to its matched static GTFS feed(s).

Existing GTFS-RT locations are preserved. If multiple static feeds are referenced, their locations are combined and deduplicated by location ID.

This addresses MobilityData/mobility-database-catalogs#1567, where recently imported STM and Metra GTFS-RT feeds had valid static feed references but empty locations.

Tests

Added regression coverage for:

• a new locationless GTFS-RT feed inheriting an existing static feed location;
• preserving an existing GTFS-RT location rather than overwriting it;
• combining locations from multiple static feed references.

Local verification:

• targeted regression tests: 3 passed;
• full api/tests/integration/populate_tests/test_populate.py: 20 passed;
• functions-python/reverse_geolocation/tests/test_reverse_geolocation_processor.py: 19 passed.

The reverse-geolocation suite was run using the repository's standard function-python-setup.sh shared/test-shared symlink setup.