fix: pagination nextCursor is empty strings (#250)

Pagination spec:
https://modelcontextprotocol.io/specification/2025-06-18/server/utilities/pagination
Pagination nextCurour compatible with None or empty strings.

Prevent reaching MAX_ITERATIONS, some third-party MCP service
implementations​. nextCursor is empty strings.

e.g., higress:


https://github.com/alibaba/higress/blob/main/plugins/golang-filter/mcp-session/common/server.go#L624-L627

Author: jianghushinian

langchain_mcp_adapters/tools.py

@@ -83,7 +83,9 @@ async def _list_all_tools(session: ClientSession) -> list[MCPTool]:
         if list_tools_page_result.tools:
             all_tools.extend(list_tools_page_result.tools)
 
-        if list_tools_page_result.nextCursor is None:
+        # Pagination spec: https://modelcontextprotocol.io/specification/2025-06-18/server/utilities/pagination
+        # compatible with None or ""
+        if not list_tools_page_result.nextCursor:
             break
 
         current_cursor = list_tools_page_result.nextCursor