fix: #2938 make agents.sandbox.sandboxes importable on Windows

[DhanushKenkiri]

PR Description

• Problem: On Windows, import agents.sandbox.sandboxes fails with ModuleNotFoundError: fcntl because the package eagerly imports the Unix-only unix_local backend (which depends on Unix-only stdlib like fcntl/termios).
• Change:
  • Guard unix_local imports in src/agents/sandbox/sandboxes/init.py so the sandboxes package can be imported on Windows even when the Unix backend is unavailable.
  • Add an explicit, user-facing ImportError in src/agents/sandbox/sandboxes/unix_local.py when imported on Windows (clearer than the current ModuleNotFoundError).
  • Add regression tests in tests/sandbox/test_sandboxes_imports.py.
• Why this approach: unix_local is inherently Unix-only; the SDK should stay importable cross-platform and only fail when a platform-incompatible backend is explicitly imported/used.
• Tests: python -m pytest -q tests/sandbox/test_sandboxes_imports.py

Fixes #2938

[seratch]

Can you resolve the lint errors?

[DhanushKenkiri]

Working on the lint error, the error is due to ruff format --check says it “would reformat” src/agents/sandbox/sandboxes/init.py. Next I’ll install the pinned Ruff version locally, run ruff format on the touched files, commit the formatting-only change, and push so the GitHub Action passes.