fix(cli): make cwd snapshot tests portable on Windows

[andrei-hasna]

Why

The rust-ci-full Windows x64 run 30317403819 exposed three deterministic codex-cli test failures. Two tests used Unix-only /target fixtures, which are not absolute Windows paths. A third compared serialized path text, so the same temporary directory failed when Windows represented it through an 8.3 short path with different casing.

What changed

• Use tempfile::TempDir for launcher and requested working directories so fixtures are absolute on every platform.
• Compare the snapshotted workspace root with paths_match_after_normalization instead of a hardcoded string.
• Inspect structured permission-profile path entries and compare filesystem identity, avoiding long-path versus short-path spelling assumptions.
• Keep production snapshot behavior unchanged; this PR changes tests only.

Verification

• just test-fast -p codex-cli runtime_context_from_config_keeps_workspace_permissions_symbolic_for_cwd_snapshot (1 passed)
• just test-fast -p codex-cli explicit_cwd_snapshot_ (2 passed)
• just test -p codex-cli agent_cmd::tests:: (9 passed)
• just fix -p codex-cli
• just fmt
• Staged and 2968352..HEAD gitleaks scans (no leaks)

The full just test-fast -p codex-cli run passed 311 of 312 tests, including all changed tests. The sole failure was the unchanged sandbox_with_network_proxy_blocks_direct_loopback_access, where local curl timed out with exit 28 instead of the expected connection-refused exit 7.

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