OAuth2 callback 500s on secrets stored by pre-1.9 versions (self-hosted upgrade ≤1.8 → 1.9.x)

# 🐛 Bug Report: OAuth2 callback 500s on secrets stored by pre-1.9 versions (self-hosted upgrade ≤1.8 → 1.9.x) ## 👟 Reproduction steps On a self-hosted instance that has **existing OAuth2 providers configured under Appwrite ≤ 1.8** (e.g. Microsoft), upgrade the server to 1.9.x (reproduced on 1.9.5) and run the migration, then attempt to sign in with that provider. 1. Configure a Microsoft (or other) OAuth2 provider on 1.8.x. 2. Upgrade to 1.9.5 (`upgrade` task + `migrate`). 3. From a client app call `createOAuth2Session` / hit `/v1/account/sessions/oauth2/microsoft`. 4. The **initiation** redirect to the provider works fine. 5. After authenticating at the provider, the **callback** returns HTTP 500 `general_unknown`. ## 👍 Expected behavior Login completes and a session is created, as it did on 1.8.x. The upgrade migration should either convert stored provider secrets to the new format, or the callback should read the legacy format like the initiation endpoint does. ## 👎 Actual behavior The callback endpoint throws: ``` TypeError: Appwrite\Auth\OAuth2::__construct(): Argument #2 ($appSecret) must be of type string, array given, called in /usr/src/code/app/controllers/api/account.php on line 1456 at /usr/src/code/src/Appwrite/Auth/OAuth2.php:43 at /usr/src/code/app/controllers/api/account.php:1456 ``` ### Root cause Pre-1.9 Appwrite stored each `<provider>Secret` inside the `oAuthProviders` map as a **nested-encrypted array** (`{data, method, iv, tag, version}`). 1.9.x stores the secret as a **plain string** in that map (with the map still encrypted at the attribute layer). - The **initiation** path (`.../oauth2/:provider/redirect`, account.php ~line 1282) decrypts the nested legacy blob before constructing the adapter — so redirects keep working. - The **callback** path (`.../oauth2/callback/...`, account.php line 1456) does **not**; it passes `oAuthProviders[$provider.'Secret']` straight into `new $className($appId, $appSecret, $callback)`. When that valu…