fix(sandbox): make typecheck pass on Windows

[rome-xi]

Summary

make typecheck reports three mypy errors on Windows and none on Linux. On win32, typeshed types tempfile.TemporaryFile() as _TemporaryFileWrapper[bytes], which is not an IOBase subclass, so strip_tar_member_prefix and Docker hydrate fail. Windows mypy also treats process as unbound after the PTY try/except/else in unix_local.

Cast the TemporaryFile objects to io.IOBase at those two sites, and create the PTY session entry inside the else branch where process is defined. No public types are widened.

Test plan

• uv run mypy --platform win32 --follow-imports=silent on the three affected modules
• the same command with --platform linux stays clean
• tests/sandbox/test_windows_tempfile_iobase.py covers the seekable-stream contract

Issue number

Fixes #4477

[seratch]

Thanks for sending this. #4499 resolves the issue and its commit includes you as a co-author.