Implement PeerMentorDetailBLoC event handlers

Use `bloc_concurrency` package: apply `restartable()` transformer to LoadPeerMentorDetail to cancel in-flight requests on new load, and `droppable()` to RefreshPeerMentorDetail to ignore taps during an active refresh. For refresh-while-loaded, emit a `PeerMentorDetailLoading` state that carries the stale model: `emit(PeerMentorDetailLoading(mentorId: event.mentorId, staleData: currentLoadedState?.data))` — the UI layer uses staleData to keep showing content behind the refresh indicator. Use `on(_onLoad, transformer: restartable())` syntax. Avoid try/catch on the entire handler body; instead use the service's typed result to branch, and only wrap in try/catch for truly unexpected exceptions that weren't handled by the service layer.

Unit tests (flutter_test + bloc_test): (1) Happy path — blocTest emits [Loading, Loaded] on LoadPeerMentorDetail with successful mock service. (2) Partial error — mock service returns partial model with error map; assert [Loading, PartialError] with correct error map. (3) Fatal error — mock service throws; assert [Loading, Error]. (4) Refresh with existing data — emit Loaded state first, then send RefreshPeerMentorDetail; assert Loading carries previous data, then Loaded.

(5) Concurrent load — send two LoadPeerMentorDetail events with different mentorIds; assert only the second one's Loaded state is emitted (droppable/restart transformer). (6) Close mid-fetch — close BLoC while fetch is in progress; assert no state emitted after close. Minimum 95% line coverage on BLoC file.