feat: require shape_id when a trip has continuous pickup or drop-off

[vssinghh]

Summary:

The spec makes trips.shape_id conditionally required: required if the trip has continuous pickup or drop-off behavior defined either in routes.txt or in stop_times.txt, optional otherwise. GtfsTripSchema already annotates shapeId as @ConditionallyRequired, but nothing enforced it.

Adds TripShapeIdConditionalValidator, which raises the existing missing_required_field notice (ERROR) on trips.shape_id when such a trip has no shape. This follows TransferStopIdsConditionalValidator and StopTimesGeographyIdPresenceValidator, which raise the generic notice under a conditional rather than defining a new notice type.

Two readings, both taken from the reference:

• 1 and empty both mean no continuous stopping behavior, so only 0, 2 and 3 make shape_id required.
• stop_times values override routes values, so the effective value for each stop time is the stop_times value where set, otherwise the route value.

Expected behavior:

Running the CLI on four variants of the Archive.zip from #1785:

routes.continuous_pickup	stop_times.continuous_pickup	result
1 (as attached)	1 (as attached)	no notice
0	1	no notice, stop_times overrides
1	0	missing_required_field on trips.txt row 2
0	empty	missing_required_field on trips.txt row 2

{"filename": "trips.txt", "csvRowNumber": 2, "fieldName": "shape_id"}

Worth noting that the archive attached to the issue has continuous_pickup=1 and continuous_drop_off=1 in every row of both files, so as attached it does not exercise the rule.

8 unit tests cover the four combinations plus the shape present, no continuous behavior, and no stop times cases.

Closes #1785

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[skalexch]

Looks good spec-wise. The tests cover all possible cases.
It would be nice to add one more granular overriding test: no shape_id + route.continuous_pickup = 0 + all stop_times have continuous_pickup=1 except for one stop_times.continuous_pickup = 2 or 3. All stop_times will override but the notice is still generated.

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[vssinghh]

Added the required test case. Thank you for the review @skalexch