Improvement on frequency based trips support

[miklcct]

Describe expected behavior

• What: We would like a full, first class support for frequency-based trips in GTFS, including real-time (UNSCHEDULED) trips support and the API to show frequency information. With NeTEx, we should also support minimum and maximum frequency as well, which does not exist in GTFS.
• Why: Some transport operations are frequency instead of timetable based. The operators do not run services to a timetable, but maintain an even headway. In extreme circumstances, the actual times can be different day by day depending on driver availability and/or passenger demand.
• When: Hopefully Q2 2027, half a year after the TfL new iBus system is rolled out

Linked issue(s)

• GraphQL Plan Query: Leg should have a headway and isNonExactFrequency response #5950
• Frequency-based trips don't appear in stoptimesForPatterns query #6422
• Fetching leg by ID doesn't work with frequency based trips #6871
• Duplicate result with unreasonable gap on itinerary search with frequency trips #6872
• Wait reluctance has no effect on frequency based trips. #6873
• Make Frequency class immutable and remove unused field #6879

OTP PO Discussion meeting details

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