Define agent integration tests framework

[kalenkevich]

Description

This PR introduces an integration testing framework for agents, enabling deterministic testing with mocked model responses.

Key Changes:

• Testing Utilities (tests/integration/agents/test_case_utils.ts):

  • Implemented MockGenAIClient and GeminiWithMockResponses to simulate LLM interactions using predefined GenerateContentResponse objects.

  • Added runTestCase helper to execute agent flows, verify expected ADK Event sequences, and validate event structure (ignoring non-deterministic fields like IDs and timestamps).

• Reference Integration Test (tests/integration/agents/simple_agent_with_tools/):

  • Created a sample LlmAgent ("weather_time_agent") with get_weather and get_current_time tools to serve as a test subject.
  • Added a Vitest test suite (agent_test.ts) that runs a multi-turn conversation.
  • Included JSON fixtures for mocked model responses and expected event outputs.

This infrastructure allows for reliable, offline integration testing of agent behaviors and tool execution flows.