feat(01-04): MsalClientFactory with per-clientId PCA and MsalCacheHelper

- Creates one IPublicClientApplication per ClientId (never shared)
- Persists token cache to configurable directory (default: %AppData%\SharepointToolbox\auth\msal_{clientId}.cache)
- SemaphoreSlim(1,1) prevents duplicate creation under concurrent calls
- CacheDirectory property exposed for test injection
- 4 unit tests: same-instance, different-instance, concurrent, AppData path

SharepointToolbox.Tests/Auth/MsalClientFactoryTests.cs

@@ -1,7 +1,75 @@
+using System;
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using SharepointToolbox.Infrastructure.Auth;
+using Xunit;
+
 namespace SharepointToolbox.Tests.Auth;
 
-public class MsalClientFactoryTests
+[Trait("Category", "Unit")]
+public class MsalClientFactoryTests : IDisposable
 {
-    [Fact(Skip = "Wave 0 stub — implemented in plan 01-04")]
-    public void CreateClient_Returns_ConfiguredPublicClientApplication() { }
+    private readonly string _tempCacheDir;
+
+    public MsalClientFactoryTests()
+    {
+        _tempCacheDir = Path.Combine(Path.GetTempPath(), "MsalClientFactoryTests_" + Guid.NewGuid());
+        Directory.CreateDirectory(_tempCacheDir);
+    }
+
+    public void Dispose()
+    {
+        try { Directory.Delete(_tempCacheDir, recursive: true); } catch { /* best effort */ }
+    }
+
+    [Fact]
+    public async Task GetOrCreateAsync_SameClientId_ReturnsSameInstance()
+    {
+        var factory = new MsalClientFactory(_tempCacheDir);
+
+        var pca1 = await factory.GetOrCreateAsync("clientA");
+        var pca2 = await factory.GetOrCreateAsync("clientA");
+
+        Assert.Same(pca1, pca2);
+    }
+
+    [Fact]
+    public async Task GetOrCreateAsync_DifferentClientIds_ReturnDifferentInstances()
+    {
+        var factory = new MsalClientFactory(_tempCacheDir);
+
+        var pcaA = await factory.GetOrCreateAsync("clientA");
+        var pcaB = await factory.GetOrCreateAsync("clientB");
+
+        Assert.NotSame(pcaA, pcaB);
+    }
+
+    [Fact]
+    public async Task GetOrCreateAsync_ConcurrentCalls_DoNotCreateDuplicateInstances()
+    {
+        var factory = new MsalClientFactory(_tempCacheDir);
+
+        // Run 10 concurrent calls with the same clientId
+        var tasks = Enumerable.Range(0, 10)
+            .Select(_ => factory.GetOrCreateAsync("clientConcurrent"))
+            .ToArray();
+
+        var results = await Task.WhenAll(tasks);
+
+        // All 10 results must be the exact same instance
+        var first = results[0];
+        Assert.All(results, r => Assert.Same(first, r));
+    }
+
+    [Fact]
+    public void CacheDirectory_ResolvesToAppData_Not_Hardcoded()
+    {
+        // The default (no-arg) constructor must use %AppData%\SharepointToolbox\auth
+        var factory = new MsalClientFactory();
+        var expectedBase = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData);
+        var expectedDir = Path.Combine(expectedBase, "SharepointToolbox", "auth");
+
+        Assert.Equal(expectedDir, factory.CacheDirectory);
+    }
 }

SharepointToolbox/Infrastructure/Auth/MsalClientFactory.cs

@@ -0,0 +1,69 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Identity.Client;
+using Microsoft.Identity.Client.Extensions.Msal;
+
+namespace SharepointToolbox.Infrastructure.Auth;
+
+/// <summary>
+/// Creates and caches one IPublicClientApplication per ClientId with a persistent MsalCacheHelper.
+/// Never shares instances across different ClientIds (per-tenant isolation).
+/// </summary>
+public class MsalClientFactory
+{
+    private readonly Dictionary<string, IPublicClientApplication> _clients = new();
+    private readonly SemaphoreSlim _lock = new(1, 1);
+
+    /// <summary>Cache directory for MSAL token files.</summary>
+    public string CacheDirectory { get; }
+
+    /// <summary>Default constructor — uses %AppData%\SharepointToolbox\auth.</summary>
+    public MsalClientFactory()
+        : this(Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+            "SharepointToolbox", "auth"))
+    { }
+
+    /// <summary>Constructor allowing a custom cache directory (used in tests).</summary>
+    public MsalClientFactory(string cacheDirectory)
+    {
+        CacheDirectory = cacheDirectory;
+    }
+
+    /// <summary>
+    /// Returns the cached IPublicClientApplication for the given clientId,
+    /// or creates a new one (with MsalCacheHelper) on first call.
+    /// Thread-safe: concurrent callers with the same clientId receive the same instance.
+    /// </summary>
+    public async Task<IPublicClientApplication> GetOrCreateAsync(string clientId)
+    {
+        await _lock.WaitAsync();
+        try
+        {
+            if (_clients.TryGetValue(clientId, out var existing))
+                return existing;
+
+            Directory.CreateDirectory(CacheDirectory);
+
+            var storageProps = new StorageCreationPropertiesBuilder(
+                    $"msal_{clientId}.cache", CacheDirectory)
+                .Build();
+
+            var pca = PublicClientApplicationBuilder
+                .Create(clientId)
+                .WithDefaultRedirectUri()
+                .WithLegacyCacheCompatibility(false)
+                .Build();
+
+            var helper = await MsalCacheHelper.CreateAsync(storageProps);
+            helper.RegisterCache(pca.UserTokenCache);
+
+            _clients[clientId] = pca;
+            return pca;
+        }
+        finally { _lock.Release(); }
+    }
+}