fix: preserve single-turn structured output

[he-yufeng]

Fixes #6089.

Summary

• Keep message_as_output for plain single-turn agent text output.
• Leave structured output_schema results as event.output instead of marking them as message output.
• Add direct tests for both plain text and structured single-turn output handling.

To verify

• $env:PYTHONPATH=(Resolve-Path .\src).Path; python -m pytest tests\unittests\workflow\test_llm_agent_as_node.py -q
• python -m py_compile src\google\adk\workflow\_llm_agent_wrapper.py tests\unittests\workflow\test_llm_agent_as_node.py
• python -m ruff check src\google\adk\workflow\_llm_agent_wrapper.py
• git diff --check