fix: add pragma to skip unreachable branch coverage

The else branch on the isinstance check is unreachable in practice
since POST SSE responses always contain JSONRPCRequest messages.

Github-Issue: #1811

Author: Jay Hemnani

src/mcp/client/streamable_http.py

@@ -438,7 +438,7 @@ async def _handle_sse_response(
             # Cannot reconnect - no event ID received before disconnection
             # Notify session layer to prevent deadlock (fixes #1811)
             logger.warning("SSE stream disconnected without resumption token, cannot reconnect")
-            if isinstance(ctx.session_message.message.root, JSONRPCRequest):
+            if isinstance(ctx.session_message.message.root, JSONRPCRequest):  # pragma: no branch
                 request_id = ctx.session_message.message.root.id
                 error_response = JSONRPCError(
                     jsonrpc="2.0",