Resolve Tests-003, -004, -005, -006 code-review findings

Tests-003: temp auth-DB directories leaked under %TEMP%. Added the
TempDatabaseDirectory IDisposable helper (clears the Sqlite connection pool,
then recursively deletes); SqliteAuthStoreTests and ApiKeyAdminCliRunnerTests
now dispose every directory they create.

Tests-004: added end-to-end coverage composing the real authorization
interceptor in front of the real MxAccessGatewayService, plus scope-resolver
tests confirming an unmapped request type fails closed to the admin scope.

Tests-005: added coverage for a worker faulting mid-command — a pipe
disconnect and a worker fault while an InvokeAsync is in flight both fail the
pending invoke. No product change needed.

Tests-006 (re-triaged): the flaky ReadLoop_WhenClientFaults_KillsOwnedWorkerProcess
is a test race, not a product bug — the kill runs synchronously inside
SetFaulted. Rewrote it to await FakeWorkerProcess exit deterministically, and
replaced fixed Task.Delay timing in the late-reply and heartbeat tests with
FIFO ordering and an injected ManualTimeProvider.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/MxGateway.Tests/Security/Authentication/SqliteAuthStoreTests.cs

@@ -11,8 +11,9 @@ namespace MxGateway.Tests.Security.Authentication;
 /// <summary>
 /// Tests for <see cref="SqliteAuthStore"/>.
 /// </summary>
-public sealed class SqliteAuthStoreTests
+public sealed class SqliteAuthStoreTests : IDisposable
 {
+    private readonly List<TempDatabaseDirectory> _tempDirectories = [];
     /// <summary>
     /// Verifies that MigrateAsync initializes the database schema.
     /// </summary>
@@ -167,12 +168,23 @@ public sealed class SqliteAuthStoreTests
         return services.BuildServiceProvider(validateScopes: true);
     }
 
-    private static string CreateTempDatabasePath()
+    /// <summary>Clears SQLite pools and deletes every temporary directory created by this test.</summary>
+    public void Dispose()
     {
-        string directory = Path.Combine(Path.GetTempPath(), "mxgateway-auth-tests", Guid.NewGuid().ToString("N"));
-        Directory.CreateDirectory(directory);
+        foreach (TempDatabaseDirectory directory in _tempDirectories)
+        {
+            directory.Dispose();
+        }
 
-        return Path.Combine(directory, "gateway-auth.db");
+        _tempDirectories.Clear();
+    }
+
+    private string CreateTempDatabasePath()
+    {
+        TempDatabaseDirectory directory = TempDatabaseDirectory.Create("mxgateway-auth-tests");
+        _tempDirectories.Add(directory);
+
+        return directory.DatabasePath();
     }
 
     private static async Task CreateVersionZeroDatabaseAsync(string databasePath)