fix: accept HTTP 201 status code in token exchange

Fixes #1502

Some OAuth servers (like Supabase) return 201 Created instead of 200 OK
for successful token exchanges. The OAuth 2.0 spec doesn't mandate a
specific success status code, so we should accept both 200 and 201.

Author: jnjpng

src/mcp/client/auth.py

@@ -429,7 +429,7 @@ async def _exchange_token(self, auth_code: str, code_verifier: str) -> httpx.Req
 
     async def _handle_token_response(self, response: httpx.Response) -> None:
         """Handle token exchange response."""
-        if response.status_code != 200:
+        if response.status_code not in {200, 201}:
             raise OAuthTokenError(f"Token exchange failed: {response.status_code}")
 
         try: