## Why
Windows CI now places temporary and build files on the `D:` Dev Drive.
Fake rollout metadata still stored `/` as its working directory, but `/`
is drive-relative on Windows. When the migrated auto-environment tests
resumed or listed those rollouts, the fixture resolved to `D:\` while
the established test expectation remained `C:\`, causing unrelated PRs
to fail the Windows app-server shard.
This follows the interaction between #31357, which moved CI build paths
to the Dev Drive, and #31614, which migrated these app-server tests to
automatic environments.
## What
- Construct fake rollout working directories with `test_path_buf("/")`,
producing a fully qualified native path on Windows while preserving `/`
on Unix.
- Use the same native test-path helper for the legacy
conversation-summary expectation.
## How to Test
Automated tests were intentionally not run locally at request; the
app-server suite was stopped during compilation. `just fmt` completed
successfully.
To verify the regression on a Windows runner:
1. Configure `TEMP` and `TMP` on a non-`C:` drive, as CI does with the
Dev Drive.
2. Run `just test -p codex-app-server`.
3. Confirm the existing thread list, read, and resume tests no longer
report `D:\` actual versus `C:\` expected paths.
This is a test-fixture-only change, so there is no product smoke path.
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