fix: ignore relative snapshot base overrides

[matthewflint]

Summary

• ignore relative XDG_STATE_HOME, LOCALAPPDATA, and APPDATA overrides
• keep the default local snapshot base dir independent from the current working directory
• add regression coverage for the relative-path cases

Testing

• uv run pytest -s tests/sandbox/test_snapshot_defaults.py
• uv run ruff check src/agents/sandbox/snapshot_defaults.py tests/sandbox/test_snapshot_defaults.py

Signed-off-by: matthewflint 277024436+matthewflint@users.noreply.github.com

[seratch]

One small concern before merging: the direction looks right, but the Windows-specific coverage is still a bit indirect.

The new tests exercise the os_name="nt" branch, but they use POSIX-style absolute paths from tmp_path. That means we are not actually proving that a Windows-style env value such as C:\Users\me\AppData\Local is treated as absolute when platform/os_name are injected for a simulated Windows case.

If platform / os_name injection is intended to support cross-platform unit tests, we may want either:

• a test that covers Windows-style absolute env paths explicitly, or
• branch-specific path handling using PureWindowsPath for the Windows env vars.

Not blocking from my side, but I think this would make the regression coverage more precise.

[matthewflint]

Yes, that makes sense. I updated the os_name == "nt" branch to use PureWindowsPath(...).is_absolute() for LOCALAPPDATA / APPDATA, and switched the tests to explicit Windows-style values like C:\Users\me\AppData\Local.

I also added coverage for APPDATA fallback, the all-relative fallback, and a guard that /tmp/... is ignored under simulated Windows.