Adapt OAuth2 implementation to Matrix Spec v1.18 (#34026)

* Adapt OAuth2 implementation to Matrix Spec v1.18

* Handle more cases of oidc->oauth

* Fix test

* Fix read back of oauth2 context

* Iterate

* Fix tests

* Discard changes to apps/web/playwright/e2e/settings/account-user-settings-tab.spec.ts

* Fix test

* Fix test

* Fix test

* Potential fix for pull request finding 'Unused variable, import, function or class'

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

* Iterate

* Iterate

* Fix test

---------

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

apps/web/test/test-utils/auth.ts

@@ -0,0 +1,9 @@
+/*
+Copyright 2024 New Vector Ltd.
+Copyright 2023 The Matrix.org Foundation C.I.C.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+export { makeDelegatedAuthMetadata } from "matrix-js-sdk/src/testing";