fix: added test helpers for platform-specific paths (#7954)

This addresses post-merge feedback from
https://github.com/openai/codex/pull/7856.

codex-rs/Cargo.lock

@@ -837,7 +837,6 @@ dependencies = [
  "codex-login",
  "codex-protocol",
  "codex-rmcp-client",
- "codex-utils-absolute-path",
  "codex-utils-json-to-toml",
  "core_test_support",
  "mcp-types",
@@ -1887,6 +1886,7 @@ dependencies = [
  "base64",
  "codex-core",
  "codex-protocol",
+ "codex-utils-absolute-path",
  "notify",
  "regex-lite",
  "serde_json",