fix: require stops.txt before GTFS batch publish

[mackenziereading19]

Summary

Ensure the GTFS batch scheduler only publishes feeds whose latest dataset contains stops.txt, while preserving first-time ingestion for feeds that do not yet have a latest dataset.

The resulting behavior is:

• feeds with no latest dataset remain eligible for publication;
• feeds whose latest dataset contains stops.txt remain eligible;
• feeds whose latest dataset exists but does not contain stops.txt are skipped.

This uses the persisted GTFS file inventory introduced by #1259 and keeps the check at the scheduler boundary before publishing the Pub/Sub message.

Tests

• focused regression covering the three cases above;
• full functions-python/batch_datasets/tests suite: 5 passed;
• Black: pass;
• Flake8 with functions-python/.flake8: pass.

Fixes #1271