feat: introduce LogLevel value object and improve logger API

- Replace LogLevel enum with a dedicated class that encapsulates
  log level representation.
- Add factory methods for creating Debug, Info, and Error levels.
- Add string conversion support through LogLevel::toString().
- Add equality comparison operators for LogLevel instances.

- Rename Logger::writeLog() to writeMessage().
- Add writeMessage() overload that uses the default log level.
- Implement actual file writing with error handling using core::Status.
- Improve logger documentation to reflect the updated API.

- Update logger tests to use the new LogLevel API.
- Move logger tests into the core test directory.
- Replace enum-based log level usage with LogLevel factory methods.
- Update test cases for the renamed writeMessage() method.

tests/core/logger_test.cpp

@@ -0,0 +1,123 @@
+#include <cassert>
+#include <fstream>
+#include <string>
+
+#include "logger.hpp"
+
+namespace {
+
+using namespace logger;
+
+void testLoggerCreation() {
+  Logger logger("test.log", LogLevel::info());
+
+  assert(logger.getFileName() == "test.log");
+  assert(logger.getLogLevel() == LogLevel::info());
+}
+
+void testWithLogLevel() {
+  Logger logger("test.log", LogLevel::info());
+
+  auto debugLogger = logger.withLogLevel(LogLevel::debug());
+
+  // Исходный объект не изменился
+  assert(logger.getLogLevel() == LogLevel::info());
+
+  // Новый объект получил новый уровень
+  assert(debugLogger.getLogLevel() == LogLevel::debug());
+
+  // Имя файла сохранилось
+  assert(debugLogger.getFileName() == "test.log");
+}
+
+void testWithFileName() {
+  Logger logger("old.log", LogLevel::info());
+
+  auto newLogger = logger.withFileName("new.log");
+
+  // Исходный объект не изменился
+  assert(logger.getFileName() == "old.log");
+
+  // Новый объект получил новый файл
+  assert(newLogger.getFileName() == "new.log");
+
+  // Уровень сохранился
+  assert(newLogger.getLogLevel() == LogLevel::info());
+}
+
+void testWriteLogSuccess() {
+  const std::string fileName = "logger_test.log";
+
+  Logger logger(fileName, LogLevel::info());
+
+  auto result = logger.writeMessage(LogLevel::info(), "Application started");
+
+  assert(result.isSuccess());
+  assert(result.getValue());
+
+  std::ifstream file(fileName);
+
+  assert(file.is_open());
+
+  std::string content;
+  std::getline(file, content);
+
+  assert(content.find("Application started") != std::string::npos);
+
+  file.close();
+
+  std::remove(fileName.c_str());
+}
+
+void testWriteLogFailure() {
+  Logger logger("/invalid/path/logger.log", LogLevel::info());
+
+  auto result = logger.writeMessage(LogLevel::info(), "message");
+
+  assert(result.isFailure());
+  assert(!result.getError().empty());
+}
+
+void testMultipleWrites() {
+  const std::string fileName = "multiple.log";
+
+  Logger logger(fileName, LogLevel::info());
+
+  assert(logger.writeMessage(LogLevel::info(), "first").isSuccess());
+
+  assert(logger.writeMessage(LogLevel::error(), "second").isSuccess());
+
+  std::ifstream file(fileName);
+
+  assert(file.is_open());
+
+  std::string line1;
+  std::string line2;
+
+  std::getline(file, line1);
+  std::getline(file, line2);
+
+  assert(line1.find("first") != std::string::npos);
+  assert(line2.find("second") != std::string::npos);
+
+  file.close();
+
+  std::remove(fileName.c_str());
+}
+
+void runTests() {
+  testLoggerCreation();
+  testWithLogLevel();
+  testWithFileName();
+  testWriteLogSuccess();
+  testWriteLogFailure();
+  testMultipleWrites();
+}
+
+} // namespace
+
+int main() {
+  runTests();
+
+  return 0;
+}