fix(genai): guard cached_content with tools

[Junaid Ahmad (junaidaslam2006)]

Summary

• Preserve list-based content: Updated for Gemini 3 and output version v1 to prevent content from collapsing into strings.
• Propagate output_version: Handled via response parsing/metadata without sending it to the request config.
• API Constraint Guard: Prevent cached_content from being combined with tools, tool_config, or system_instruction to comply with Gemini API requirements.

Fixes #1528

Testing

I have verified the fix using the following integration tests:

pytest libs/genai/tests/integration_tests/test_chat_models.py::test_search_builtin
pytest libs/genai/tests/integration_tests/test_chat_models.py::test_structured_output_with_google_search
pytest libs/genai/tests/integration_tests/test_chat_models.py::test_gemini_3_pro_agent_loop_streaming
pytest libs/genai/tests/integration_tests/test_standard.py::TestGeminiProStandardGoogleAI::test_structured_output_optional_param

[Junaid Ahmad (junaidaslam2006)]

llm test is failing
can any maintainer rerun and share last test name