test: cover empty tool results

[nightcityblade]

Related Issues

• Fixes Add regression coverage for ToolInvoker with empty tool results #12455

Proposed Changes:

Add parameterized regression coverage for tools that successfully return an empty string, list, or dictionary. The tests exercise both synchronous and asynchronous invocation and verify serialization, success status, and tool-call correlation.

How did you test it?

• hatch run test:unit test/components/agents/test_tool_calling.py -q — 71 passed
• hatch run pre-commit run --files test/components/agents/test_tool_calling.py — all hooks passed
• hatch run fmt test/components/agents/test_tool_calling.py — passed, unchanged

Notes for the reviewer

This is test-only regression coverage; runtime behavior is unchanged.

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes through this linked PR.
• I have added unit tests; docstring updates are N/A because no production API changed.
• I've used the test: conventional commit type for my PR title.
• Documentation is N/A because this PR changes regression tests only.
• A release note is N/A because this test-only PR has no user-facing change.
• I have run pre-commit hooks and all applicable hooks passed.

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