feat: Add conditional min_transfer_time validation

[mackenziereading19]

Summary:

Implements #2174 by adding validation for the conditionally required transfers.min_transfer_time field when transfer_type = 2 (MINIMUM_TIME).

Expected behavior:

When transfer_type = 2 and min_transfer_time is missing or empty, the validator emits the existing missing_required_field ERROR notice for min_transfer_time in transfers.txt.

A supplied value of 0 remains valid. Other transfer types do not require min_transfer_time.

Implementation:

• Mark min_transfer_time as @ConditionallyRequired in GtfsTransferSchema.
• Add TransferMinTransferTimeConditionalValidator.
• Reuse the existing MissingRequiredFieldNotice.
• Add focused unit tests covering missing, zero, positive, and non-MINIMUM_TIME cases.

Validation:

• Focused validator tests: 4/4 passed.
• Existing transfer validation tests: 3/3 passed.
• End-to-end CLI validation reproduced the expected missing_required_field notice.
• Spotless formatting check passed.
• Full :main:test was run. Three unrelated Mockito-based tests also fail on the clean parent commit, confirming they are pre-existing and not introduced by this change.
• No documentation changes are required for this focused validator.
• Linked issue: Add notice for conditionally required min_transfer_time #2174.
• Screenshots: not applicable for this validator change.

Closes #2174

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[CLAassistant]

All committers have signed the CLA.

[skalexch]

Looks good logic and test-wise.