ConnectivityInterceptor: inject ConnectivityService for testability

[cevheri]

Problem

ConnectivityInterceptor at connectivity_interceptor.dart:24 uses the ConnectivityService singleton directly, making it impossible to test the offline rejection branch deterministically.

Fix

Accept a ConnectivityService (or status provider interface) via constructor with a default of ConnectivityService.instance, so tests can supply a fake.

Source

Copilot review on PR #59