Define MentorLocationService interface and failure types

Use Dart's abstract interface class keyword (Dart 3+) rather than abstract class to signal that this is a pure interface with no default implementations. Define the sealed failure hierarchy in a separate file mentor_location_failure.dart alongside the interface. Choose Either over exceptions for all async methods — this forces callers (BLoCs) to handle failures explicitly and aligns with functional patterns common in Flutter BLoC projects. If the project does not yet use fpdart/dartz, define a lightweight Result type locally to avoid the dependency.

The FakeMentorLocationService should expose a call log so BLoC tests can assert fetchMentorsInBoundingBox was called with the expected arguments.

No tests required for the interface itself. However, create FakeMentorLocationService in test/fakes/fake_mentor_location_service.dart that implements MentorLocationService with configurable responses (e.g., setNextResponse(Either result)).

This fake will be used by all downstream BLoC tests. Write one smoke test confirming FakeMentorLocationService correctly implements all three method signatures at compile time.