Feat: Add /login/channels route and improve auth logic for frontend third-party login integration (#7521)

### What problem does this PR solve?

Add `/login/channels` route and improve auth logic to support frontend
integration with third-party login providers:

- Add `/login/channels` route to provide authentication channel list
with `display_name` and `icon`
- Optimize user info parsing logic by prioritizing `avatar_url` and
falling back to `picture`
- Simplify OIDC token validation by removing unnecessary `kid` checks
- Ensure `client_id` is safely cast to string during `audience`
validation
- Fix typo

---
- Related pull request: #7379 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

api/apps/user_app.py

@@ -116,7 +116,30 @@ def login():
         )
 
 
-@manager.route("/login/<channel>") # noqa: F821
+@manager.route("/login/channels", methods=["GET"]) # noqa: F821
+def get_login_channels():
+    """
+    Get all supported authentication channels.
+    """
+    try:
+        channels = []
+        for channel, config in settings.OAUTH_CONFIG.items():
+            channels.append({
+                "channel": channel,
+                "display_name": config.get("display_name", channel.title()),
+                "icon": config.get("icon", "sso"),
+            })
+        return get_json_result(data=channels)
+    except Exception as e:
+        logging.exception(e)
+        return get_json_result(
+            data=[],
+            message=f"Load channels failure, error: {str(e)}",
+            code=settings.RetCode.EXCEPTION_ERROR
+        )
+
+
+@manager.route("/login/<channel>", methods=["GET"]) # noqa: F821
 def oauth_login(channel):
     channel_config = settings.OAUTH_CONFIG.get(channel)
     if not channel_config:
@@ -171,7 +194,7 @@ def oauth_callback(channel):
                 users = user_register(
                     user_id,
                     {
-                        "access_token": access_token,
+                        "access_token": get_uuid(),
                         "email": user_info.email,
                         "avatar": avatar,
                         "nickname": user_info.nickname,
@@ -189,7 +212,7 @@ def oauth_callback(channel):
                 # Try to log in
                 user = users[0]
                 login_user(user)
-                return redirect(f"/?auth_success=true&user_id={user.get_id()}")
+                return redirect(f"/?auth={user.get_id()}")
 
             except Exception as e:
                 rollback_user_registration(user_id)
@@ -201,8 +224,9 @@ def oauth_callback(channel):
         user.access_token = get_uuid()
         login_user(user)
         user.save()
-        return redirect(f"/?auth_success=true&user_id={user.get_id()}")
+        return redirect(f"/?auth={user.get_id()}")
     except Exception as e:
+        logging.exception(e)
         return redirect(f"/?error={str(e)}")