test(app-server): use native rollout fixture paths

[fcoury-oai]

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.