[codex] Fix marketplace paths on non-Windows platforms

[bc19sam-afk]

What changed

• write native local marketplace paths on macOS and Linux
• keep the existing \\?\ extended-path behavior on Windows
• treat Windows-prefixed paths as invalid on non-Windows systems so existing malformed entries are repaired
• update marketplace tests to assert platform-specific paths
• add a regression test for repairing \\?\/Users/...-style paths on Unix

Root cause

windows_extended_path() was used unconditionally when writing local marketplace sources. On macOS this converted paths such as:

/Users/name/.codex/.tmp/plugins

into:

\\?\/Users/name/.codex/.tmp/plugins

The resulting path does not point to the marketplace directory on macOS, so Codex reports that the marketplace root has no supported manifest. The comparison logic also stripped the prefix on every platform, which prevented Codex++ from repairing already-malformed non-Windows entries.

Related to #1169.

Impact

After this change, launching Codex++ or repairing the plugin marketplace no longer corrupts marketplace paths on macOS/Linux. Existing malformed paths are corrected on the next marketplace configuration pass. Windows behavior remains unchanged.

Validation

• cargo fmt --all -- --check
• git diff --check
• cargo test -p codex-plus-core plugin_marketplace::tests — 12 passed
• cargo test -p codex-plus-core --lib — 113 passed

The broader cargo test -p codex-plus-core run reaches an unrelated existing failure in codex_process_filter_keeps_chatgpt_desktop_package_processes. The same failure reproduces on an unmodified upstream/main worktree.