fix: Export Backoff class publicly to resolve implementation_imports error

- Add Backoff to public exports in launchdarkly_event_source_client.dart
- Update import in streaming_data_source.dart to use public API instead of internal /src path
- Fixes dart analyzer implementation_imports violation that was causing CI failures

Co-Authored-By: [email protected] <[email protected]>

Author: devin-ai-integration[bot]

packages/common_client/lib/src/data_sources/streaming_data_source.dart

@@ -5,7 +5,6 @@ import 'package:http/http.dart' as http;
 
 import 'package:launchdarkly_dart_common/launchdarkly_dart_common.dart';
 import 'package:launchdarkly_event_source_client/launchdarkly_event_source_client.dart';
-import 'package:launchdarkly_event_source_client/src/backoff.dart';
 
 import '../config/data_source_config.dart';
 import '../config/defaults/credential_type.dart';

packages/event_source_client/lib/launchdarkly_event_source_client.dart

@@ -16,6 +16,7 @@ export 'src/events.dart' show Event, MessageEvent, OpenEvent;
 export 'src/test_sse_client.dart' show TestSseClient;
 export 'src/logging.dart'
     show EventSourceLogger, LogLevel, NoOpLogger, PrintLogger;
+export 'src/backoff.dart' show Backoff;
 
 /// HTTP methods supported by the event source client.
 enum SseHttpMethod {