feat(eval): expose user_simulator_config in generate_responses

[primenko-v]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

N/A

2. Or, if no issue exists, describe the change:

Problem:
EvaluationGenerator.generate_responses constructs a UserSimulatorProvider() with no arguments, so the LLM-backed path always runs with the default BaseUserSimulatorConfig. There is no way for a caller to override the user-simulation model, max-allowed invocations, or custom instructions when driving multi-turn conversations through LlmBackedUserSimulator.

Solution:
Add an optional user_simulator_config parameter to generate_responses and forward it to UserSimulatorProvider(...). Callers can now pass an LlmBackedUserSimulatorConfig to customize the LLM-backed simulator.

The behavior is backward compatible:

• When the argument is omitted, UserSimulatorProvider falls back to BaseUserSimulatorConfig() exactly as before.
• Static eval cases are unaffected: the config is ignored by StaticUserSimulator.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

A unit test for the proposed change was added to tests/unittests/evaluation/test_evaluation_generator.py: TestGenerateResponses::test_generate_responses_forwards_llm_backed_user_simulator_config

All tests pass:

> uv run pytest tests/unittests/ -rs
...
================================== short test summary info ================================== 
SKIPPED [1] tests/unittests/integrations/crewai/test_crewai_tool.py:20: Requires Python 3.10+
================  5770 passed, 1 skipped, 2358 warnings in 129.40s (0:02:09) ================

The skipped test is not related to this change — it skips on main as well.

Manual End-to-End (E2E) Tests:

A reference setup lives at https://github.com/primenko-v/adk-x-mlflow (tag pr-demo/user-simulator-config).

It loads an LlmBackedUserSimulatorConfig from YAML and forwards it to EvaluationGenerator.generate_responses via the new user_simulator_config parameter — see src/mlflow_adk/simulate.py.

To reproduce (requires GOOGLE_CLOUD_PROJECT and ADC via gcloud auth application-default login):

git clone --recurse-submodules --branch pr-demo/user-simulator-config \
    https://github.com/primenko-v/adk-x-mlflow.git
cd adk-x-mlflow
cp .env.example .env  # fill in GOOGLE_CLOUD_PROJECT
uv sync
uv run python -m mlflow_adk.simulate --no-mlflow --output-traces traces.jsonl

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.