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'

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* Iterate

* Iterate

* Fix test

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apps/web/test/unit-tests/components/structures/LoggedInView-test.tsx

@@ -57,6 +57,7 @@ describe("<LoggedInView />", () => {
         setExtendedProfileProperty: jest.fn().mockResolvedValue(undefined),
         deleteExtendedProfileProperty: jest.fn().mockResolvedValue(undefined),
         doesServerSupportExtendedProfiles: jest.fn().mockResolvedValue(true),
+        getAuthMetadata: jest.fn().mockRejectedValue(new Error("Legacy auth")),
     });
     const mediaHandler = new MediaHandler(mockClient);
     const mockSdkContext = new TestSDKContext();