Fix/route agency db error

[FLASH2332]

Problem:
In routeHandler, the error returned by GetAgency was silently discarded:
goagency, err := api.GtfsManager.GtfsDB.Queries.GetAgency(ctx, agencyID)

if err == nil {
    // agency added to references
}
// any error is silently dropped

If this DB call failed for any reason — including context.Canceled or context.DeadlineExceeded — the handler would skip the agency reference and return a successful response with incomplete data.

Fix :
sql.ErrNoRows → agency reference is optional, skip silently and continue
Any other error → route through serverErrorResponse, which correctly handles context cancellation and deadline exceeded

Changes :
internal/restapi/route_handler.go — added explicit error handling for GetAgency

Closes #792

[fletcherw]

Hi Jayadev, nice work noticing this issue. When I query the public API with an agency ID that doesn't exist I get a NotFound error, so I think the maglev implementation of routes should do the same.

As part of this fix, you should also add a test for this behavior in route_handler_test.

curl https://api.pugetsound.onebusaway.org/api/where/route/2_100224.json\?key\=TEST
{"code":404,"currentTime":1774819186552,"text":"resource not found","version":2}%