test(logging): add logging tests

apps/test/core/logging/log_entry_test.dart

@@ -0,0 +1,84 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:social_app/core/logging/log_entry.dart';
+
+void main() {
+  group('LogEntry', () {
+    test('toJson should serialize correctly', () {
+      final entry = LogEntry(
+        timestamp: DateTime(2026, 4, 1, 10, 0, 0),
+        level: LogLevel.error,
+        message: 'Test error',
+        module: 'test.module',
+        funcName: 'testFunc',
+        lineNo: 42,
+        errorType: 'TestError',
+        stackTrace: 'test stack',
+        extra: {'key': 'value'},
+      );
+
+      final json = entry.toJson();
+
+      expect(json['level'], 'error');
+      expect(json['error_type'], 'TestError');
+      expect(json['extra'], {'key': 'value'});
+    });
+
+    test('toConsoleString should format correctly', () {
+      final entry = LogEntry(
+        timestamp: DateTime(2026, 4, 1, 10, 0, 0),
+        level: LogLevel.debug,
+        message: 'Test message',
+        module: 'test.module',
+        funcName: 'testFunc',
+        lineNo: 42,
+      );
+
+      final result = entry.toConsoleString();
+
+      expect(result, contains('DEBUG'));
+      expect(result, contains('test.module'));
+      expect(result, contains('testFunc'));
+      expect(result, contains('@42'));
+      expect(result, contains('Test message'));
+    });
+
+    test('toFileString should format correctly', () {
+      final entry = LogEntry(
+        timestamp: DateTime(2026, 4, 1, 10, 0, 0),
+        level: LogLevel.error,
+        message: 'Test error',
+        module: 'test.module',
+        funcName: 'testFunc',
+        lineNo: 42,
+        errorType: 'TestError',
+        stackTrace: 'test stack trace',
+      );
+
+      final result = entry.toFileString();
+
+      expect(result, contains('[test.module]'));
+      expect(result, contains('at testFunc:42'));
+      expect(result, contains('Test error'));
+      expect(result, contains('Error: TestError'));
+      expect(result, contains('StackTrace:'));
+      expect(result, contains('test stack trace'));
+    });
+
+    test('toJson should omit null fields', () {
+      final entry = LogEntry(
+        timestamp: DateTime(2026, 4, 1, 10, 0, 0),
+        level: LogLevel.info,
+        message: 'Test',
+        module: 'test',
+      );
+
+      final json = entry.toJson();
+
+      expect(json.containsKey('func_name'), false);
+      expect(json.containsKey('line_no'), false);
+      expect(json.containsKey('error_type'), false);
+      expect(json.containsKey('stack_trace'), false);
+      expect(json.containsKey('extra'), false);
+    });
+  });
+}