fix(tool): resolve default failure handler at invoke time

[OiPunk]

Summary

Fixes #945 by resolving the default tool failure handler at invocation time instead of binding it at decorator definition time.

This allows instrumentation libraries (for example Sentry) to patch default_tool_error_function after import and still have function tools use the patched handler.

What changed

• Introduced an internal sentinel for function_tool(..., failure_error_function=...) to distinguish:
  • omitted argument (use default handler)
  • explicit None (re-raise exceptions)
  • explicit custom handler
• Updated runtime error handling in function_tool to resolve default_tool_error_function inside the invocation path.
• Added regression test proving that monkey-patching default_tool_error_function after creating the tool still affects tool failure handling.

Tests

• uv run ruff check src/agents/tool.py tests/test_function_tool.py
• uv run mypy src/agents/tool.py tests/test_function_tool.py
• uv run pytest tests/test_function_tool.py -q
• make format-check
• uv run python -m trace --count --coverdir /tmp/openai_agents_945_trace.Ig3U8E --module pytest tests/test_function_tool.py -q

Coverage note: changed branches in src/agents/tool.py are all hit by tests (including omitted/default, explicit None, and explicit custom handler paths).

[OiPunk]

Follow-up fix pushed in 6e00ced for CI old_version_tests.

Root cause:

• A runtime cast(ToolErrorFunction | None, ...) expression introduced by this PR is not valid on Python 3.9, and raised:
  TypeError: unsupported operand type(s) for |: '_CallableGenericAlias' and 'NoneType'.

Fix:

• Replaced the runtime cast target with Optional[ToolErrorFunction] for py3.9 compatibility.

Local verification after the fix:

• uv run ruff check src/agents/tool.py tests/test_function_tool.py
• uv run mypy src/agents/tool.py tests/test_function_tool.py
• uv run pytest tests/test_function_tool.py tests/test_function_tool_decorator.py -q
• env -u all_proxy -u ALL_PROXY -u http_proxy -u HTTP_PROXY -u https_proxy -u HTTPS_PROXY -u NO_PROXY -u no_proxy UV_PROJECT_ENVIRONMENT=.venv_39 uv run --python 3.9 -m pytest tests/test_function_tool.py tests/test_function_tool_decorator.py tests/test_run_step_execution.py tests/test_tracing_errors.py tests/test_tracing_errors_streamed.py -k "simple_function or complex_args_function or custom_error_function or async_failure_error_function_is_awaited or invalid_json_raises_with_failure_error_function_none or tool_call_error or error_on_invalid_json or no_error_on_invalid_json" -q