fix: expose original FunctionTool callable

[he-yufeng]

Summary

Adds a stable FunctionTool.func property for tools created with @function_tool.

The original callable was previously only reachable through private closure-walking on the generated invoker. This stores it on the tool as internal _func metadata and exposes it through a read-only property, so downstream SDKs and tests can introspect the wrapped function without depending on closure internals.

This intentionally does not make FunctionTool callable and does not use functools.update_wrapper. PR #2146 showed that wrapper metadata can interact badly with pytest collection for tool functions whose names start with test_; this version keeps the runtime shape unchanged except for the new property.

Test plan

• python -m py_compile src\agents\tool.py tests\test_function_tool.py
• python -m ruff check src\agents\tool.py tests\test_function_tool.py
• python -m ruff format --check src\agents\tool.py tests\test_function_tool.py
• $env:PYTHONPATH='src'; python -m pytest tests\test_function_tool.py -q
• $env:PYTHONPATH='src'; python -c "from agents import function_tool; add=lambda a,b=1:a+b; tool=function_tool(add); assert tool.func is add; assert not callable(tool); print('ok')"

Issue number

Closes #3381

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass