Implement MentorMapBloc with dual state machines

Model MentorMapState as a single immutable class with two fields: mapView (MapViewState) and filterPanel (FilterPanelState), both sealed classes. Use copyWith to produce the next state, which keeps event handlers concise. For debounce, use EventTransformer from the bloc_concurrency package (restartable transformer) on the BoundingBoxChanged event handler — this also cancels in-flight fetches automatically. Do not use two separate BLoCs; keeping both state machines in one BLoC avoids cross-BLoC synchronisation complexity and ensures atomic state transitions.

Use equatable on all state and event classes. Keep MentorLocationService injection via constructor for testability. The BLoC should not know about the map widget library (e.g., flutter_map or google_maps) — all geographic types should be project-defined (BoundingBox, LatLng) so the BLoC remains UI-framework agnostic.

Use bloc_test package for all BLoC tests. Write an expect sequence test for each event type covering: the initial state, intermediate states (e.g., MapViewLoading emitted before MapViewLoaded), and the final steady state. Test debounce by emitting multiple rapid BoundingBoxChanged events and asserting that MentorLocationService.fetchMentorsInBoundingBox is called only once. Test concurrent event handling: emit FiltersApplied during an in-flight BoundingBoxChanged fetch and verify the BLoC does not emit inconsistent states.

Test error propagation: mock MentorLocationService to throw a NetworkException and verify MapViewError is emitted. Test cache hit: mock service to return CacheHitState and verify lastUpdated is present in MapViewLoaded. Minimum 95% branch coverage on event handlers.