Python: fix: preserve explicit null tool arguments

[he-yufeng]

Summary

• preserve explicitly provided null tool arguments after Pydantic validation
• cover both direct FunctionTool.invoke() and automatic function-call execution
• keep the existing exclude_none behavior for fields that were not explicitly supplied

Fixes #5934.

Validation

python -m pytest tests/core/test_tools.py::test_tool_invoke_preserves_required_nullable_argument tests/core/test_function_invocation_logic.py::test_base_client_with_function_calling_preserves_null_arguments -q -p no:cacheprovider --basetemp .tmp\pytest-null-args
python -m pytest tests/core/test_tools.py -q -p no:cacheprovider --basetemp .tmp\pytest-tools
python -m ruff check packages/core/agent_framework/_tools.py packages/core/tests/core/test_tools.py packages/core/tests/core/test_function_invocation_logic.py
python -m ruff format --check packages/core/agent_framework/_tools.py packages/core/tests/core/test_tools.py packages/core/tests/core/test_function_invocation_logic.py
python -m py_compile packages/core/agent_framework/_tools.py packages/core/tests/core/test_tools.py packages/core/tests/core/test_function_invocation_logic.py
python -m pyright packages/core/agent_framework/_tools.py packages/core/tests/core/test_tools.py packages/core/tests/core/test_function_invocation_logic.py

[Copilot]

Pull request overview

This PR fixes a Python tool-invocation bug where explicitly provided null/None arguments were being dropped after Pydantic validation, causing required-but-nullable tool parameters to be omitted (notably in auto function-calling flows).

Changes:

• Introduces _dump_tool_arguments() to keep exclude_none=True behavior while re-including explicitly-supplied None fields.
• Updates FunctionTool.invoke() and the auto function-call execution path to use _dump_tool_arguments().
• Adds regression tests covering both direct FunctionTool.invoke() and automatic function-call execution.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
python/packages/core/agent_framework/_tools.py	Adds _dump_tool_arguments() and uses it in tool invocation and auto function-call execution to preserve explicit null arguments.
python/packages/core/tests/core/test_tools.py	Adds a regression test ensuring direct invoke() preserves explicitly provided None for a required nullable parameter.
python/packages/core/tests/core/test_function_invocation_logic.py	Adds a regression test ensuring auto function calling preserves null arguments from model-generated function calls.