Add notice for conditionally required min_transfer_time

[skalexch]

Describe the problem

#2172

In google/transit#640, the spec was corrected to make sure that min_transfer_time becomes required for timed transfers (transfer_type = 2).

Describe the new validation rule

Related spec language	Rationale for rule	Notice name to update or add	Pseudologic	Notice table columns	Severity	Notice description
transfers.transfer_type, transfers.min_transfer_time	transfer_type=2 is defined as "Transfer requires a minimum amount of time between arrival and departure to ensure a connection. " Therefore, min_transfer_time is required in this case.	missing_required_field	If transfer_type == 2 and min_transfer_time is empty or missing --> Trigger Notice	csvRowNumber, fieldName (min_transfer_time), filename (transfers.txt)	ERROR	The given field has no value in some input row, even though values are required.

Sample GTFS datasets

No response

Severity

ERROR

Additional context

No response

[mackenziereading19]

Hi! I'd be interested in working on this once it's been triaged. I've done some initial investigation locally and confirmed that a transfer_type=2 record with an empty min_transfer_time currently does not produce missing_required_field. The generated GtfsTransfer entity exposes hasMinTransferTime(), so this looks suitable for a focused conditional validator using the existing notice. Happy to take this on if that approach sounds appropriate.

[emmambd]

Thanks very much @mackenziereading19! This has been defined thanks to @skalexch and is ready to be worked on.