fix: support PathLike values in file tuples

[MukundaKatta]

Summary:

• stop treating tuples as raw FileContent in _files.py so tuple uploads reach the tuple transformation path
• support PathLike values inside multipart file tuples as documented by the type hints
• add sync and async regression tests for tuple PathLike inputs with an explicit content type

Testing:

• PYTHONPATH=src python3 - <<'PY'
  from pathlib import Path
  from anthropic._files import to_httpx_files
  readme = Path('README.md').resolve()
  result = to_httpx_files({'file': ('README.md', readme, 'text/markdown')})
  assert result['file'][0] == 'README.md'
  assert result['file'][1] == readme.read_bytes()
  assert result['file'][2] == 'text/markdown'
  print('sync ok')
  PY
• PYTHONPATH=src python3 - <<'PY'
  import asyncio
  from pathlib import Path
  from anthropic._files import async_to_httpx_files
  readme = Path('README.md').resolve()
  async def main():
  result = await async_to_httpx_files({'file': ('README.md', readme, 'text/markdown')})
  assert result['file'][0] == 'README.md'
  assert result['file'][1] == readme.read_bytes()
  assert result['file'][2] == 'text/markdown'
  print('async ok')
  asyncio.run(main())
  PY

Fixes #1318