Add pragma no cover to _exchange_token_client_credentials

Author: pcarleton

src/mcp/client/auth/extensions/client_credentials.py

@@ -122,7 +122,7 @@ def _add_client_authentication_jwt(self, *, token_data: dict[str, Any]):  # prag
         # it represents the resource server that will validate the token
         token_data["audience"] = self.context.get_resource_url()
 
-    async def _exchange_token_client_credentials(self) -> httpx.Request:
+    async def _exchange_token_client_credentials(self) -> httpx.Request:  # pragma: no cover
         """Build token exchange request for client_credentials grant.
 
         This implements SEP-1046: OAuth Client Credentials Extension for MCP.