fix(docs): Use @tool decorator for retriever in agentic RAG tutorial

[Shrey327]

Overview

Fixed the agentic RAG tutorial to use the @tool decorator instead of create_retriever_tool. The previous approach caused a TypeError when used with LangGraph's ToolNode because create_retriever_tool returns a functools.partial object, which is incompatible with typing.get_type_hints().

Type of change

Type: Fix typo/bug/link/formatting

Related issues/PRs

• GitHub issue: N/A
• Feature PR: N/A

Checklist

• I have read the contributing guidelines
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

Additional notes

The issue manifests as: TypeError: functools.partial(...) is not a module, class, method, or function during graph compilation/execution. The @tool decorator creates a proper StructuredTool that ToolNode can correctly inspect.