fix: require transfers stop ids when transfer_type is empty

[yasumorishima]

Summary:

Closes #2105.

The spec change in google/transit@a806576 (google/transit#591) made transfers.from_stop_id and transfers.to_stop_id Required if transfer_type is empty, 0, 1, 2 or 3 — previously the reference only required them for 1, 2 and 3.

TransferStopIdsConditionalValidator wraps its per-row check in if (transfer.hasTransferType()), so rows whose transfer_type is empty are skipped entirely and the newly required case is never validated. An empty transfer_type is a recommended transfer point, which is the same as transfer_type=0 — a value the validator already treats as requiring both stop ids.

This drops that guard so every row is checked. In-seat transfer types (4 and 5) keep their exemption through the existing isTransferTypeInSeat check, so the only behavior that changes is the empty case.

Expected behavior:

transfers.txt row	before	after
transfer_type empty, no from_stop_id / to_stop_id	no notice	two missing_required_field notices, one per field
transfer_type empty, both stop ids present	no notice	no notice
transfer_type 0–3, stop ids missing	two notices	unchanged
transfer_type 4 or 5, stop ids missing	no notice	unchanged

No new notice type is introduced — this reuses missing_required_field — so RULES.md needs no change.

Two unit tests are added for the empty transfer_type case, which the existing tests never covered (every existing case calls setTransferType()).

One note on CI: I ran the checks on my fork first. Format code, Test Package Document (test on Java 17 for both ubuntu-latest and windows-latest) and Package Installers are green. Rule acceptance tests cannot run on a fork — its get-reports matrix requires the ubuntu-latest-4-cores runner label, which forks do not have — so that comparison needs to run here. Since this rule newly fires on real feeds that leave transfer_type empty, the acceptance report is worth a look before merging.

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 — no documentation change needed, see above
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
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[skalexch]

Looks good on my side!