feat: add WeChat OAuth provider support

[lyj0309]

What kind of change does this PR introduce?
Feature - New WeChat OAuth provider support

What is the current behavior?
The authentication service currently supports various external OAuth providers, but WeChat OAuth integration was missing. WeChat is a popular social platform in China that uses a different OAuth flow compared to standard providers like Google or GitHub.

What is the new behavior?
This PR adds comprehensive WeChat OAuth provider support with the following features:

WeChat OAuth Authorization Flow: Implements the complete WeChat OAuth 2.0 flow including authorization and token exchange
User Identity Handling: Since WeChat doesn't provide email addresses, the system generates synthetic emails using the format {openid}@wechat.local
UnionID Support: Handles WeChat's unique UnionID feature for cross-app user identification
Comprehensive Test Coverage: Includes extensive test cases covering:
Standard signup flow
Disabled signup scenarios (with and without existing users)
Invite token validation
User ban handling
Token error scenarios
WeChat-specific metadata handling (openid, unionid, avatar, etc.)
Key Implementation Details:

Uses openid as the primary identifier since WeChat doesn't provide emails
Stores WeChat-specific metadata including unionid, openid, nickname, and avatar
Properly handles WeChat's API endpoints (/sns/oauth2/access_token and /sns/userinfo)
Implements proper error handling for WeChat API responses
Additional context
This implementation follows WeChat's official OAuth 2.0 documentation and handles the unique aspects of WeChat's authentication system, such as:

Using appid instead of client_id
Supporting both openid (app-specific) and unionid (cross-app) identifiers
Handling WeChat's specific scope snsapi_login
Managing WeChat's user profile data structure
The test suite ensures robust coverage of all edge cases and error scenarios specific to WeChat OAuth integration.

ref https://developers.weixin.qq.com/doc/oplatform/en/Website_App/WeChat_Login/Wechat_Login.html