fix: reject json response mime with function calling

[he-yufeng]

Summary

• reject response_mime_type="application/json" when an agent has tools and function calling is enabled
• keep the reported working path (function_calling_config.mode="NONE") allowed
• add regression coverage for AUTO, ANY, implicit function calling, and NONE

Fixes #5735.

Validation

• .\.venv\Scripts\python.exe -m pytest tests\unittests\agents\test_llm_agent_fields.py -q -k "json_response_with_function_calling or validate_generate_content_config"
• .\.venv\Scripts\python.exe -m pytest tests\unittests\flows\llm_flows\test_basic_processor.py tests\unittests\flows\llm_flows\test_output_schema_processor.py -q
• .\.venv\Scripts\pyink.exe --check src\google\adk\agents\llm_agent.py tests\unittests\agents\test_llm_agent_fields.py
• git diff --check

Note: direct single-file mypy src/google/adk/agents/llm_agent.py still reports existing strict typing issues elsewhere in that file, outside this change.