Preparing and upgrade of FlightService

- [ ] Create interface for FlightService methods
- [ ] Create UpgradedFlightService and implement the interface
- [ ] Implement a useClass operator for the FlightService and provider the new service
- [ ] Change URLSearchParams to HttpParams
- [ ] Remove map to json
- [ ] Apply types to the new HttpClient methods
- [ ] Handle Errors

Start by locating the existing FlightService and its HTTP usage; the issue names no files or tests. Work through the checklist items, then verify that the upgraded service uses the new HttpClient APIs, has typed requests, and handles errors without the old mapping behavior.

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