fix: mark feeds without service dates inactive

[mackenziereading19]

Summary

Fixes #1209.

Feeds whose latest GTFS dataset has no derived service-date range are now classified as inactive, matching the maintainer-confirmed behaviour in #1209.

The update removes the query filter that previously excluded datasets with null service-date bounds, then treats a null start or end date as inactive before evaluating the existing future/active/expired conditions.

Tests

Added a database-backed regression covering a feed whose latest dataset has null service-date bounds.

Verification performed locally against the repository test database:

• new null-service-date regression: passed twice
• full update_feed_status_main.py test file: 6 passed, run twice
• existing failure-path mock updated to use SQLAlchemy column expressions after the query shape changed
• final diff limited to functions-python/helpers/feed_status.py and functions-python/update_feed_status/tests/test_update_feed_status_main.py

Commit: 5d26c41fd01e44f61775c77ad2b7919eeccc96f7