docs: add pre/post processing docs for ADK Python

[twishabansal]

Description

Created again on accidental deletion of #2433

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🛠️ Fixes #<issue_number_goes_here>

[gemini-code-assist]

Code Review

This pull request introduces Python examples for pre- and post-processing using ADK, which is a valuable addition. The documentation has been updated and tests have been improved to be data-driven. However, there are a few issues in the new example code and documentation that need attention. Specifically, there are logical errors in the example test cases for both ADK and LangChain, where the conditions to trigger the pre-processing logic are not met. Additionally, a significant portion of the 'Best Practices' documentation has been removed, and it would be beneficial to clarify the reasoning behind this change.

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docs/en/samples/pre_post_processing/python/adk/agent.py (131)

This test case is intended to demonstrate a policy violation for a stay longer than 14 days, which is checked in the enforce_business_rules pre-processing callback. However, this callback only triggers for the update-hotel tool. The user input 'Book a hotel...' will likely call the book-hotel tool instead. To correctly test the pre-processing logic, the user input should be phrased as an update.

        "Update my booking for hotel 5 with checkin date 2025-01-18 and checkout date 2025-02-10",

docs/en/samples/pre_post_processing/python/langchain/agent.py (97)

This test case is meant to test the pre-processing logic that prevents hotel stays longer than 14 days. However, the checkout date has been changed to '2025-01-20', making the stay only 2 days long. This no longer triggers the policy violation. To correctly demonstrate the pre-processing rule, the checkout date should result in a stay longer than 14 days, for example, by reverting to '2025-02-20'.

        user_input = "Update my hotel with id 3 with checkin date 2025-01-18 and checkout date 2025-02-20."

docs/en/samples/pre_post_processing/_index.md (53-79)

This change removes the entire 'Best Practices' section, which contained valuable information on security, observability, performance, and error handling. Was this removal intentional? If this content is being moved to another page, it would be helpful to include a link. Otherwise, please consider restoring this section as it provides important guidance for developers.

docs/en/samples/pre_post_processing/python/adk/agent.py (16)

There's a grammatical error in the system prompt. it's name should be its name.

  cancellations. When the user searches for a hotel, mention its name, id,

docs/en/samples/pre_post_processing/python/adk/agent.py (26-28)

There's a typo in the function name. It should be enforce_business_rules. Please also update its usage on line 112.

async def enforce_business_rules(
    tool: ToolboxTool, args: Dict[str, Any], tool_context: ToolContext
) -> Optional[Dict[str, Any]]:

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