fix: Improve tracing shutdown safety

[seratch]

This pull request resolves the potential errors with tracing providers when streams are already closed:

--- Logging error ---
Traceback (most recent call last):
  File "uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/logging/__init__.py", line 1163, in emit
    stream.write(msg + self.terminator)
ValueError: I/O operation on closed file.
Call stack:
  File "openai-agents-python/src/agents/tracing/provider.py", line 309, in shutdown
    logger.debug("Shutting down trace provider")
Message: 'Shutting down trace provider'
Arguments: ()
--- Logging error ---
Traceback (most recent call last):
  File "uv/python/cpython-3.12.9-macos-aarch64-none/lib/python3.12/logging/__init__.py", line 1163, in emit
    stream.write(msg + self.terminator)
ValueError: I/O operation on closed file.
Call stack:
  File "openai-agents-python/src/agents/tracing/provider.py", line 310, in shutdown
    self._multi_processor.shutdown()
  File "openai-agents-python/src/agents/tracing/provider.py", line 86, in shutdown
    logger.debug(f"Shutting down trace processor {processor}")
Message: 'Shutting down trace processor <tests.testing_processor.SpanProcessorForTests object at 0x10d54dca0>'
Arguments: ()