feat: Add LangGraph Restaurant Finder sample agent

[faizan842]

Description

This PR adds a new sample agent (samples/agent/langgraph/restaurant_finder) implemented using LangGraph.

This sample demonstrates how to build A2UI-compliant applications using the LangGraph framework, replicating the functionality of the existing ADK restaurant finder. It provides a reference architecture for handling the A2UI protocol (UI generation, event handling, and data updates) within a LangGraph state machine.

Key improvements included in this implementation:

• Robust JSON Handling: Includes validation logic to auto-repair common LLM syntax errors (e.g. unquoted keys).
• Correct Event Parsing: Updates the executor logic to correctly handle book_restaurant and submit_booking events.
• Stateless Design: Configured without a checkpointer to handle independent A2A server requests cleanly.

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation (Added README.md for the sample).
• My code changes (if any) have tests (Manually verified determinism and logic).

If you need help, consider asking for advice on the discussion board.

[gemini-code-assist]

Code Review

This pull request introduces a new sample agent for a restaurant finder using LangGraph. The implementation is comprehensive, covering the agent logic, server setup, and UI examples. My review focuses on improving code quality, fixing a critical issue with a hardcoded URL that would affect functionality, and ensuring the sample is easy for developers to use. I've provided suggestions for refactoring to handle state correctly, cleaning up unused code and developer comments, and aligning the project's configuration with its documentation.

[gemini-code-assist]

The base_url is hardcoded within the search_restaurants tool. This will cause incorrect image URLs to be generated if the server is run on a different host or port. The base_url is available in the AgentState, but the tool, being globally defined, cannot access it.

To fix this, you should refactor the graph and tool creation to be stateful, for instance, by moving them inside the RestaurantAgent class. This would allow the tool to be created with access to the base_url from the class instance.

[gemini-code-assist]

The import RestaurantAgent is not used in this file and should be removed. The associated comment appears to be a developer note that is no longer relevant.

Additionally, there are other developer comments that should be cleaned up to improve code clarity:

• Lines 66-68: Comment about hardcoding SUPPORTED_CONTENT_TYPES.
• Lines 103-105: Comment about the images directory.

[gemini-code-assist]

This file contains several unused imports that should be removed for code clarity: os, Any, Dict, ToolMessage, ChatPromptTemplate, MessagesPlaceholder, and RunnableConfig. Additionally, MemorySaver is imported on line 192 but is also unused.

Suggested change

	import json
	import logging
	import os
	from typing import Annotated, Any, Dict, List, TypedDict
	import jsonschema
	from langchain_core.messages import BaseMessage, HumanMessage, AIMessage, ToolMessage
	from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder
	from langchain_core.tools import tool
	from langchain_google_genai import ChatGoogleGenerativeAI
	from langchain_core.runnables import RunnableConfig
	from langgraph.graph import END, StateGraph, START
	from langgraph.graph.message import add_messages
	from langgraph.prebuilt import ToolNode
	import json
	import logging
	from typing import Annotated, List, TypedDict
	import jsonschema
	from langchain_core.messages import BaseMessage, HumanMessage, AIMessage
	from langchain_core.tools import tool
	from langchain_google_genai import ChatGoogleGenerativeAI
	from langgraph.graph import END, StateGraph, START
	from langgraph.graph.message import add_messages
	from langgraph.prebuilt import ToolNode

[gemini-code-assist]

The import re statement is inside the validate_response function. According to the PEP 8 style guide, imports should be at the top of the file. Please move this import to the top-level of the module.

[gemini-code-assist]

The requires-python is set to >=3.13, but the README.md for this sample specifies Python 3.9 or higher. These should be consistent to avoid confusion for users trying to run the sample. Given that Python 3.13 is a very recent release, you might want to verify if the code can run on an earlier, more common version (like 3.9+) and update this requirement accordingly for broader compatibility.

[gemini-code-assist]

The get_restaurants function's tool_context: ToolContext parameter introduces an unnecessary dependency on the ADK (google.adk.tools.tool_context). This requires creating a MockToolContext in agent.py just to pass the base_url.

To simplify and make the code more self-contained within the LangGraph context, consider passing base_url directly as a string argument. This would also require updating the implementation to use the base_url string directly instead of tool_context.state.get("base_url").

Suggested change

	def get_restaurants(cuisine: str, location: str, tool_context: ToolContext, count: int = 5) -> str:
	def get_restaurants(cuisine: str, location: str, base_url: str, count: int = 5) -> str: