Create sessions through the user session manager

Sign-in now creates and stores its session through the
UserSessionManagerProtocol rather than the UserSessionStore, so the
AppCoordinator no longer holds the store.

- The manager forwards clientSessionDelegate and
  userSession(for:sessionDirectories:passphrase:) to its store. As with
  restoring, the new session isn't registered until the AppCoordinator
  adds it.
- AuthenticationService takes the manager, for both sign-in and soft
  logout.
- The store mock's configuration moves to the manager mock.

ElementX/Sources/Mocks/UserSessionManagerMock.swift

@@ -0,0 +1,17 @@
+//
+// Copyright 2026 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+extension UserSessionManagerMock {
+    struct Configuration { }
+    
+    convenience init(_ configuration: Configuration) {
+        self.init()
+        
+        userSessionForSessionDirectoriesPassphraseReturnValue = .success(UserSessionMock(.init()))
+        clientSessionDelegate = KeychainControllerMock()
+    }
+}