fix: schedule-for-route stop groupings, service IDs, and consolidated GTFS double prefix

[Ahmedhossamdev]

Fixes:
#789
#790
#791

• stopTripGroupings direction order, stop ordering, and headsigns didn't match Java OBA.
  Extracted shared groupTripsByDirection helper used by both stops-for-route and
  schedule-for-route.

• serviceIds was returning all active agency service IDs for the day instead of only the
  ones used by trips on the queried route.

• Double agency prefix on IDs from consolidated GTFS feeds (e.g. Puget Sound).
  FormCombinedID now skips prefixing when the ID already starts with {agencyID}_
  or normalizes {agencyID}- → {agencyID}_ to avoid 40_40-455 style output.

[github-actions]

Performance Smoke Test Results

Status: PASSED

Metric	Value
p(95) latency	2.3 ms
Error rate	0.00%
Total requests	331
Req/sec	10.9

Smoke test config: 5 VUs x 30s. Thresholds: p(95) < 300ms, error rate < 1%.

Full results uploaded as workflow artifact: k6-smoke-summary.