Update ADK doc according to issue #1587 - 11

[adk-bot]

11. Eval Scenario Generation

Doc file: docs/evaluate/user-sim.md

Current state:

The docs/evaluate/user-sim.md document explains how to add eval cases manually using adk eval_set add_eval_case but does not mention automated generation.

Proposed Change:

Add documentation for the new adk eval_set generate_eval_cases command. Explain that this command uses the Vertex AI Eval SDK (ScenarioGenerator) to dynamically generate a suite of conversation scenarios and automatically adds them to the specified eval set based on a user simulation config file. Provide an example of how to use it: adk eval_set generate_eval_cases <agent_path> <eval_set_id> --user_simulation_config_I file <path>.

Reasoning:
This is a powerful new CLI capability that automates the tedious process of writing manual conversation scenarios for evaluation by leveraging an LLM to generate them.

Reference: src/google/adk/cli/cli_tools_click.py

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