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/ConfigFileService.cs

@@ -0,0 +1,75 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using JdeScoping.ConfigManager.Models;
+using Microsoft.Extensions.Logging;
+
+namespace JdeScoping.ConfigManager.Services;
+
+/// <summary>
+/// Service for loading and saving configuration files.
+/// </summary>
+public class ConfigFileService : IConfigFileService
+{
+    private readonly IFileSystem _fileSystem;
+    private readonly ILogger<ConfigFileService>? _logger;
+
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        WriteIndented = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+        PropertyNameCaseInsensitive = true
+    };
+
+    public ConfigFileService(IFileSystem fileSystem, ILogger<ConfigFileService>? logger = null)
+    {
+        _fileSystem = fileSystem;
+        _logger = logger;
+    }
+
+    public async Task<ConfigModel> LoadAppSettingsAsync(string path, CancellationToken ct = default)
+    {
+        _logger?.LogInformation("Loading appsettings from {Path}", path);
+
+        try
+        {
+            var json = await _fileSystem.ReadAllTextAsync(path, ct);
+            var config = JsonSerializer.Deserialize<ConfigModel>(json, JsonOptions);
+            return config ?? new ConfigModel();
+        }
+        catch (JsonException ex)
+        {
+            throw new ConfigLoadException(path, $"Failed to parse appsettings.json: {ex.Message}", ex);
+        }
+    }
+
+    public async Task<PipelinesConfigModel> LoadPipelinesAsync(string path, CancellationToken ct = default)
+    {
+        _logger?.LogInformation("Loading pipelines from {Path}", path);
+
+        try
+        {
+            var json = await _fileSystem.ReadAllTextAsync(path, ct);
+            var config = JsonSerializer.Deserialize<PipelinesConfigModel>(json, JsonOptions);
+            return config ?? new PipelinesConfigModel();
+        }
+        catch (JsonException ex)
+        {
+            throw new ConfigLoadException(path, $"Failed to parse pipelines.json: {ex.Message}", ex);
+        }
+    }
+
+    public async Task SaveAppSettingsAsync(string path, ConfigModel config, CancellationToken ct = default)
+    {
+        _logger?.LogInformation("Saving appsettings to {Path}", path);
+        var json = JsonSerializer.Serialize(config, JsonOptions);
+        await _fileSystem.WriteAllTextAsync(path, json, ct);
+    }
+
+    public async Task SavePipelinesAsync(string path, PipelinesConfigModel config, CancellationToken ct = default)
+    {
+        _logger?.LogInformation("Saving pipelines to {Path}", path);
+        var json = JsonSerializer.Serialize(config, JsonOptions);
+        await _fileSystem.WriteAllTextAsync(path, json, ct);
+    }
+}

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

@@ -0,0 +1,15 @@
+namespace JdeScoping.ConfigManager.Services;
+
+/// <summary>
+/// Exception thrown when configuration file loading fails.
+/// </summary>
+public class ConfigLoadException : Exception
+{
+    public string FilePath { get; }
+
+    public ConfigLoadException(string filePath, string message, Exception? inner = null)
+        : base(message, inner)
+    {
+        FilePath = filePath;
+    }
+}

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);
+}

NEW/tests/JdeScoping.ConfigManager.Tests/Services/ConfigFileServiceTests.cs

@@ -0,0 +1,54 @@
+using JdeScoping.ConfigManager.Models;
+using JdeScoping.ConfigManager.Services;
+
+namespace JdeScoping.ConfigManager.Tests.Services;
+
+public class ConfigFileServiceTests
+{
+    private readonly IFileSystem _fileSystem;
+    private readonly ConfigFileService _sut;
+
+    public ConfigFileServiceTests()
+    {
+        _fileSystem = Substitute.For<IFileSystem>();
+        _sut = new ConfigFileService(_fileSystem);
+    }
+
+    [Fact]
+    public async Task LoadAppSettingsAsync_WithValidJson_ReturnsConfigModel()
+    {
+        // Arrange
+        var json = """
+        {
+            "DataSync": {
+                "Enabled": true,
+                "MaxDegreeOfParallelism": 8
+            }
+        }
+        """;
+        _fileSystem.ReadAllTextAsync(Arg.Any<string>(), Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult(json));
+
+        // Act
+        var result = await _sut.LoadAppSettingsAsync("/config/appsettings.json");
+
+        // Assert
+        result.ShouldNotBeNull();
+        result.DataSync.Enabled.ShouldBeTrue();
+        result.DataSync.MaxDegreeOfParallelism.ShouldBe(8);
+    }
+
+    [Fact]
+    public async Task LoadAppSettingsAsync_WithInvalidJson_ThrowsWithHelpfulMessage()
+    {
+        // Arrange
+        var json = "{ invalid json }";
+        _fileSystem.ReadAllTextAsync(Arg.Any<string>(), Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult(json));
+
+        // Act & Assert
+        var ex = await Should.ThrowAsync<ConfigLoadException>(
+            () => _sut.LoadAppSettingsAsync("/config/appsettings.json"));
+        ex.Message.ShouldContain("parse");
+    }
+}