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

[rohityan]

Hi @he-yufeng , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Can you please fix the failing unit tests before we can proceed with the review.