docs: add pre/post processing docs for ADK Python (googleapis#2534)

## Description

Created again on accidental deletion of
googleapis#2433

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🛠️ Fixes #<issue_number_goes_here>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: twishabansal

docs/en/samples/pre_post_processing/python.md

@@ -16,7 +16,14 @@ This guide demonstrates how to implement these patterns in your Toolbox applicat
 
 {{< tabpane persist=header >}}
 {{% tab header="ADK" text=true %}}
-Coming soon.
+The following example demonstrates how to use `ToolboxToolset` with ADK's pre and post processing hooks to implement pre and post processing for tool calls.
+
+```py
+{{< include "python/adk/agent.py" >}}
+```
+You can also add model-level (`before_model_callback`, `after_model_callback`) and agent-level (`before_agent_callback`, `after_agent_callback`) hooks to intercept messages at different stages of the execution loop. 
+
+For more information, see the [ADK Callbacks documentation](https://google.github.io/adk-docs/callbacks/types-of-callbacks/).
 {{% /tab %}}
 {{% tab header="Langchain" text=true %}}
 The following example demonstrates how to use `ToolboxClient` with LangChain's middleware to implement pre- and post- processing for tool calls.

docs/en/samples/pre_post_processing/python/adk/agent.py

@@ -0,0 +1,137 @@
+import asyncio
+from datetime import datetime
+from typing import Any, Dict, Optional
+from copy import deepcopy
+
+from google.adk import Agent
+from google.adk.apps import App
+from google.adk.runners import Runner
+from google.adk.sessions.in_memory_session_service import InMemorySessionService
+from google.adk.tools.tool_context import ToolContext
+from google.genai import types
+from toolbox_adk import CredentialStrategy, ToolboxToolset, ToolboxTool
+
+SYSTEM_PROMPT = """
+  You're a helpful hotel assistant. You handle hotel searching, booking and
+  cancellations. When the user searches for a hotel, mention it's name, id,
+  location and price tier. Always mention hotel ids while performing any
+  searches. This is very important for any operations. For any bookings or
+  cancellations, please provide the appropriate confirmation. Be sure to
+  update checkin or checkout dates if mentioned by the user.
+  Don't ask for confirmations from the user.
+"""
+
+
+# Pre processing
+async def enfore_business_rules(
+    tool: ToolboxTool, args: Dict[str, Any], tool_context: ToolContext
+) -> Optional[Dict[str, Any]]:
+    """
+    Callback fired before a tool is executed.
+    Enforces business logic: Max stay duration is 14 days.
+    """
+    tool_name = tool.name
+    print(f"POLICY CHECK: Intercepting '{tool_name}'")
+
+    if tool_name == "update-hotel" and "checkin_date" in args and "checkout_date" in args:
+        start = datetime.fromisoformat(args["checkin_date"])
+        end = datetime.fromisoformat(args["checkout_date"])
+        duration = (end - start).days
+
+        if duration > 14:
+            print("BLOCKED: Stay too long")
+            return {"result": "Error: Maximum stay duration is 14 days."}
+    return None
+
+
+# Post processing
+async def enrich_response(
+    tool: ToolboxTool,
+    args: Dict[str, Any],
+    tool_context: ToolContext,
+    tool_response: Any,
+) -> Optional[Any]:
+    """
+    Callback fired after a tool execution.
+    Enriches response for successful bookings.
+    """
+    if isinstance(tool_response, dict):
+        result = tool_response.get("result", "")
+    elif isinstance(tool_response, str):
+        result = tool_response
+    else:
+        return None
+
+    tool_name = tool.name
+    if isinstance(result, str) and "Error" not in result:
+        if tool_name == "book-hotel":
+            loyalty_bonus = 500
+            enriched_result = f"Booking Confirmed!\n You earned {loyalty_bonus} Loyalty Points with this stay.\n\nSystem Details: {result}"
+
+            if isinstance(tool_response, dict):
+                modified_response = deepcopy(tool_response)
+                modified_response["result"] = enriched_result
+                return modified_response
+            else:
+                return enriched_result
+    return None
+
+
+async def run_chat_turn(
+    runner: Runner, session_id: str, user_id: str, message_text: str
+):
+    """Executes a single chat turn and prints the interaction."""
+    print(f"\nUSER: '{message_text}'")
+    response_text = ""
+    async for event in runner.run_async(
+        user_id=user_id,
+        session_id=session_id,
+        new_message=types.Content(role="user", parts=[types.Part(text=message_text)]),
+    ):
+        if event.content and event.content.parts:
+            for part in event.content.parts:
+                if part.text:
+                    response_text += part.text
+
+    print(f"AI: {response_text}")
+
+
+async def main():
+    toolset = ToolboxToolset(
+        server_url="http://127.0.0.1:5000",
+        toolset_name="my-toolset",
+        credentials=CredentialStrategy.toolbox_identity(),
+    )
+    tools = await toolset.get_tools()
+    root_agent = Agent(
+        name="root_agent",
+        model="gemini-2.5-flash",
+        instruction=SYSTEM_PROMPT,
+        tools=tools,
+        # add any pre and post processing callbacks
+        before_tool_callback=enfore_business_rules,
+        after_tool_callback=enrich_response,
+    )
+    app = App(root_agent=root_agent, name="my_agent")
+    runner = Runner(app=app, session_service=InMemorySessionService())
+    session_id = "test-session"
+    user_id = "test-user"
+    await runner.session_service.create_session(
+        app_name=app.name, user_id=user_id, session_id=session_id
+    )
+
+    # First turn: Successful booking
+    await run_chat_turn(runner, session_id, user_id, "Book hotel with id 3.")
+    print("-" * 50)
+    # Second turn: Policy violation (stay > 14 days)
+    await run_chat_turn(
+        runner,
+        session_id,
+        user_id,
+        "Book a hotel with id 5 with checkin date 2025-01-18 and checkout date 2025-02-10",
+    )
+    await toolset.close()
+
+
+if __name__ == "__main__":
+    asyncio.run(main())

docs/en/samples/pre_post_processing/python/adk/requirements.txt

@@ -0,0 +1,3 @@
+google-adk[toolbox]==1.23.0
+toolbox-adk==0.5.8
+google-genai==1.62.0