fix(oauth): forward hook errors and sync linked profiles on rewritten callback (#9702, #9788, #8758)

Re-applies three main fixes onto next's rewritten OAuth callback, which
now routes generic-oauth through the shared callback route.

- #9702/#9788: a before-callback hook that rejects sign-in (the banned-user
  guard is the flagship case) now redirects to the per-flow errorURL
  carrying the hook's machine-readable code and message, instead of
  surfacing a raw error response. handleOAuthUserInfo is wrapped in
  try/catch; the hook's APIError code is forwarded verbatim (it is
  app-defined, not a member of OAUTH_CALLBACK_ERROR_CODES) via next's
  local redirectOnError closure. The admin plugin's stale inline /callback
  redirect is removed so the banned-user path flows through that single
  forwarder and emits BANNED_USER with a URL-encoded description.
- #8758: accountLinking.updateUserInfoOnLink now applies on the explicit
  OAuth link path, not only id-token linking.

The regression tests (banned social sign-in emits error=BANNED_USER, the
cross-origin errorCallbackURL case, and the 403-JSON id-token case) arrived
via the merge and were red on the baseline; these source changes make them
pass.

(cherry picked from commit 1d893ae572277447025e29462946a7aeb11443ac)

packages/better-auth/src/api/routes/callback.ts

@@ -6,10 +6,14 @@ import { getAwaitableValue } from "../../context/helpers";
 import { setSessionCookie } from "../../cookies";
 import { OAUTH_CALLBACK_ERROR_CODES } from "../../oauth2/error-codes";
 import { missingEmailLogMessage } from "../../oauth2/errors";
-import { handleOAuthUserInfo } from "../../oauth2/link-account";
+import {
+	applyUpdateUserInfoOnLink,
+	handleOAuthUserInfo,
+} from "../../oauth2/link-account";
 import { generateState, parseState } from "../../oauth2/state";
 import { setTokenUtil } from "../../oauth2/utils";
 import { HIDE_METADATA } from "../../utils/hide-metadata";
+import { isAPIError } from "../../utils/is-api-error";
 
 const schema = z.object({
 	code: z.string().optional(),
@@ -274,6 +278,7 @@ export const callbackOAuth = createAuthEndpoint(
 					);
 				}
 			}
+			await applyUpdateUserInfoOnLink(c, link.userId, userInfo);
 			let toRedirectTo: string;
 			try {
 				const url = callbackURL;
@@ -294,20 +299,33 @@ export const callbackOAuth = createAuthEndpoint(
 			...tokens,
 			scope: tokens.scopes?.join(","),
 		};
-		const result = await handleOAuthUserInfo(c, {
-			userInfo: {
-				...userInfo,
-				id: providerAccountId,
-				email: userInfo.email,
-				name: userInfo.name || "",
-			},
-			account: accountData,
-			callbackURL,
-			disableSignUp:
-				(provider.disableImplicitSignUp && !requestSignUp) ||
-				provider.options?.disableSignUp,
-			overrideUserInfo: provider.options?.overrideUserInfoOnSignIn,
-		});
+		let result: Awaited<ReturnType<typeof handleOAuthUserInfo>>;
+		try {
+			result = await handleOAuthUserInfo(c, {
+				userInfo: {
+					...userInfo,
+					id: providerAccountId,
+					email: userInfo.email,
+					name: userInfo.name || "",
+				},
+				account: accountData,
+				callbackURL,
+				disableSignUp:
+					(provider.disableImplicitSignUp && !requestSignUp) ||
+					provider.options?.disableSignUp,
+				overrideUserInfo: provider.options?.overrideUserInfoOnSignIn,
+			});
+		} catch (e) {
+			// A before-callback hook (for example the admin plugin's banned-user
+			// guard) can reject sign-in by throwing an APIError. Forward its
+			// machine-readable code and message to the per-flow errorURL instead
+			// of surfacing a raw error response. The code is app-defined, so it is
+			// forwarded verbatim rather than mapped onto OAUTH_CALLBACK_ERROR_CODES.
+			if (isAPIError(e) && e.body?.code) {
+				redirectOnError(e.body.code, e.body.message);
+			}
+			throw e;
+		}
 		if (result.error) {
 			c.context.logger.error(result.error.split(" ").join("_"));
 			return redirectOnError(result.error.split(" ").join("_"));

packages/better-auth/src/plugins/admin/admin.ts

@@ -113,15 +113,6 @@ export const admin = <O extends AdminOptions>(options?: O | undefined) => {
 											return;
 										}
 
-										if (ctx.path.startsWith("/callback")) {
-											const redirectURI =
-												ctx.context.options.onAPIError?.errorURL ||
-												`${ctx.context.baseURL}/error`;
-											throw ctx.redirect(
-												`${redirectURI}?error=banned&error_description=${opts.bannedUserMessage}`,
-											);
-										}
-
 										throw APIError.from("FORBIDDEN", {
 											message: opts.bannedUserMessage,
 											code: "BANNED_USER",