Fix LangChain load_tools import in smolagents tools docs

[jaluik]

Summary

• update the smolagents tools docs to import load_tools from langchain_community.agent_toolkits.load_tools
• add the SerpAPI Python package to the English install command
• instantiate InferenceClientModel() in the LangChain examples so the snippets are self-contained
• sync the same import/model snippet fix across translated tools docs

Verification

• git diff --check
• rg -n "from langchain\.agents import load_tools" units || true
• verified from langchain_community.agent_toolkits.load_tools import load_tools and load_tools(["serpapi"])[0] in a temporary Python environment