fix: override GTFS-RT entity types during populate

[mackenziereading19]

Summary

Fixes #1812.

GTFS-RT entity types were previously added to the existing relationship during catalogue population rather than replacing the previous catalogue values.

This meant that if a feed changed from, for example:

• sa|vp
• to sa

the stored result remained sa|vp.

This change clears the existing entity-type relationship before applying a non-empty catalogue entity_type value, so the latest catalogue row replaces the previous values.

Regression coverage

Added an integration regression using the existing layered populate fixture:

• earlier populate row: sa|vp
• later populate row: sa
• expected final API value: ["sa"]

Before the fix, the test failed with:

['sa', 'vp'] != ['sa']

After the fix, it passes.

Tests

• focused regression: 1 passed
• populate integration package: 21 passed
• relevant API/model tests: 130 passed

Empty entity_type cell semantics are intentionally unchanged by this patch.