Fix: preserve existing refresh_token when server omits it in refresh response

Per RFC 6749 Section 6, the server MAY issue a new refresh token.
If the response omits it, preserve the existing one to prevent
can_refresh_token() from returning False after first refresh.

Fixes #2270

Author: Jah-yee

src/mcp/client/auth/oauth2.py

@@ -458,6 +458,17 @@ async def _handle_refresh_response(self, response: httpx.Response) -> bool:  # p
             content = await response.aread()
             token_response = OAuthToken.model_validate_json(content)
 
+            # Per RFC 6749 Section 6, the server MAY issue a new refresh token.
+            # If the response omits it, preserve the existing one.
+            if (
+                not token_response.refresh_token
+                and self.context.current_tokens
+                and self.context.current_tokens.refresh_token
+            ):
+                token_response = token_response.model_copy(
+                    update={"refresh_token": self.context.current_tokens.refresh_token}
+                )
+
             self.context.current_tokens = token_response
             self.context.update_token_expiry(token_response)
             await self.context.storage.set_tokens(token_response)