Add support for Agent Engine Sessions

[AmaadMartin]

PR Description: Add support for Agent Engine Sessions

This PR implements the VertexAiSessionService to integrate with Vertex AI Agent Engine Sessions, addressing the requirements in Buganizer task 494654280.

Changes

• Core: Implemented VertexAiSessionService in core/src/sessions/vertex_ai_session_service.ts and registered it in registry.ts and index.ts.
• Tests:
  • Added unit tests in core/test/sessions/vertex_ai_session_service_test.ts.
  • Added integration test in tests/integration/tools/agent_tool_test.ts.
  • Added E2E test in tests/e2e/context_compaction/e2e_compaction_test.ts.
• Configuration: Updated vitest.config.ts and package.json as needed.

Verification

• Unit tests for VertexAiSessionService passed successfully.
• Integration test for AgentTool passed successfully.
• End-to-End test for context compaction passed successfully against a live Reasoning Engine in project adk-kokoro.

[kalenkevich]

can you please create new test instead of modifying existing one, thanks

[kalenkevich]

same as here: https://github.com/google/adk-js/pull/249/changes#r3066181278

[kalenkevich]

same https://github.com/google/adk-js/pull/249/changes#r3066181278