[codex] Tighten external migration prompt tests

[alexsong-oai]

Summary

• use platform-specific project path fixtures in external_agent_config_migration prompt tests
• keep the existing POSIX snapshot for non-Windows test runs
• add a Windows-specific snapshot for the prompt rendering on Windows

Why

The previous test fixture used POSIX-style absolute project paths and implicitly relied on display_path_for() normalizing them to project-relative paths. That assumption held for the non-Windows snapshot, but it did not hold for the failing Windows Cargo run, which rendered absolute project paths instead.

This change keeps the test coverage focused on the actual behavior of each platform by feeding Windows-style project paths on Windows and POSIX-style project paths elsewhere.

Validation

• just fmt
• attempted cargo test -p codex-tui external_agent_config_migration::tests:: locally, but the build did not complete before I pushed this update

Notes

Per the repo instructions, I did not rerun tests after the final just fmt pass.