feat(configmanager): add ConfigFileService with tests

Add config file loading and saving service following TDD approach:
- IConfigFileService interface for loading/saving config files
- ConfigLoadException for descriptive error handling
- ConfigFileService implementation with JSON serialization
- Unit tests with mocked IFileSystem dependency

NEW/src/Utils/JdeScoping.ConfigManager/Services/IConfigFileService.cs

@@ -0,0 +1,14 @@
+using JdeScoping.ConfigManager.Models;
+
+namespace JdeScoping.ConfigManager.Services;
+
+/// <summary>
+/// Service for loading and saving configuration files.
+/// </summary>
+public interface IConfigFileService
+{
+    Task<ConfigModel> LoadAppSettingsAsync(string path, CancellationToken ct = default);
+    Task<PipelinesConfigModel> LoadPipelinesAsync(string path, CancellationToken ct = default);
+    Task SaveAppSettingsAsync(string path, ConfigModel config, CancellationToken ct = default);
+    Task SavePipelinesAsync(string path, PipelinesConfigModel config, CancellationToken ct = default);
+}