fix: load artifacts from workflow text responses

[he-yufeng]

Summary

• support load_artifacts results that were converted into workflow context text
• scan all parts in the latest content turn instead of assuming the first part is the function response
• add regressions for workflow text responses and non-first function responses

Fixes #5967.

To verify

• .\.venv\Scripts\python -m pytest tests\unittests\tools\test_load_artifacts_tool.py -q
• .\.venv\Scripts\python -m pyink --check src\google\adk\tools\load_artifacts_tool.py tests\unittests\tools\test_load_artifacts_tool.py
• .\.venv\Scripts\python -m py_compile src\google\adk\tools\load_artifacts_tool.py tests\unittests\tools\test_load_artifacts_tool.py
• git diff --check

[adk-bot]

Response from ADK Triaging Agent

Hello @he-yufeng, thank you for creating this PR!

We noticed that a few requirements from our contribution guidelines are missing:

1. Summary of passed unit tests: Please run your tests and include a summary of the passed pytest results in your PR description.
2. Logs or screenshots: Please provide execution logs or screenshots showing the tool in action after applying the fix to help reviewers better understand the change.

Providing these details will help the maintainers review and merge your PR more efficiently. Thank you!