feat: modify stops.txt to be conditionally required

[cka-y]

Summary:

Modifies stops.txt to be conditionally required.
Logic: If stops.txt doesn't exist, only trigger a missing_required_file notice if locations.geojson also doesn't exist.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[github-actions]

📝 Acceptance Test Report

📋 Summary

✅ The rule acceptance has passed for commit 25b0c72
Download the full acceptance test report here (report will disappear after 90 days).

📊 Notices Comparison

New Errors (0 out of 1588 datasets, ~0%) ✅

No changes were detected due to the code change.

Dropped Errors (0 out of 1588 datasets, ~0%) ✅

No changes were detected due to the code change.

New Warnings (0 out of 1588 datasets, ~0%) ✅

No changes were detected due to the code change.

Dropped Warnings (0 out of 1588 datasets, ~0%) ✅

No changes were detected due to the code change.

🛡️ Corruption Check

0 out of 1588 sources (~0 %) are corrupted.

⏱️ Performance Assessment

📈 Validation Time

Assess the performance in terms of seconds taken for the validation process.

Time Metric	Dataset ID	Reference (s)	Latest (s)	Difference (s)
Average	--	4.03	4.07	⬆️+0.03
Median	--	1.42	1.48	⬆️+0.06
Standard Deviation	--	11.24	11.28	⬆️+0.03
Minimum in References Reports	ar-buenos-aires-subterraneos-de-buenos-aires-subte-gtfs-6	0.51	0.66	⬆️+0.16
Maximum in Reference Reports	gb-unknown-uk-aggregate-feed-gtfs-2014	285.87	288.37	⬆️+2.50
Minimum in Latest Reports	us-california-city-of-wasco-gtfs-1788	0.53	0.55	⬆️+0.01
Maximum in Latest Reports	gb-unknown-uk-aggregate-feed-gtfs-2014	285.87	288.37	⬆️+2.50

[github-actions]

📝 Acceptance Test Report

📋 Summary

✅ The rule acceptance has passed for commit 238cdd7
Download the full acceptance test report here (report will disappear after 90 days).

📊 Notices Comparison

New Errors (0 out of 1588 datasets, ~0%) ✅

No changes were detected due to the code change.

Dropped Errors (0 out of 1588 datasets, ~0%) ✅

No changes were detected due to the code change.

New Warnings (0 out of 1588 datasets, ~0%) ✅

No changes were detected due to the code change.

Dropped Warnings (0 out of 1588 datasets, ~0%) ✅

No changes were detected due to the code change.

🛡️ Corruption Check

0 out of 1588 sources (~0 %) are corrupted.

⏱️ Performance Assessment

📈 Validation Time

Assess the performance in terms of seconds taken for the validation process.

Time Metric	Dataset ID	Reference (s)	Latest (s)	Difference (s)
Average	--	4.03	4.07	⬆️+0.04
Median	--	1.43	1.47	⬆️+0.04
Standard Deviation	--	11.46	11.45	⬇️-0.01
Minimum in References Reports	us-california-flex-v2-developer-test-feed-2-gtfs-1818	0.51	0.55	⬆️+0.05
Maximum in Reference Reports	gb-unknown-uk-aggregate-feed-gtfs-2014	296.41	297.26	⬆️+0.85
Minimum in Latest Reports	us-oregon-hut-airport-shuttle-gtfs-635	0.68	0.52	⬇️-0.16
Maximum in Latest Reports	gb-unknown-uk-aggregate-feed-gtfs-2014	296.41	297.26	⬆️+0.85

[qcdyx]

LGTM