fix(oauth_proxy): 🐛 add _extra_token_params as kwargs in refresh_token call (#2387)

custom oauth providers sometimes require extra parameters in their token calls, hence the need for the _extra_token_params in the inital token request. This PR includes those extra token params in the token refresh request too.

src/fastmcp/server/auth/oauth_proxy.py

@@ -1326,6 +1326,7 @@ class OAuthProxy(OAuthProvider):
                 url=self._upstream_token_endpoint,
                 refresh_token=upstream_token_set.refresh_token,
                 scope=" ".join(scopes) if scopes else None,
+                **self._extra_token_params,
             )
             logger.debug("Successfully refreshed upstream token")
         except Exception as e: