fix(macos): preserve app icons during entrypoint repair

[shaoxianbilly]

Summary

• make repaired macOS app bundles reference the packaged .icns icon
• copy the .icns from the current app bundle instead of looking for a missing PNG sidecar
• verify in CI that every CFBundleIconFile actually exists
• add regression assertions for repaired bundle metadata

Root cause

The release DMG is packaged correctly with codex-plus-plus.icns, but the in-app entrypoint repair path rewrites Info.plist to codex-plus-plus.png and looks for a PNG beside the executable. That file is absent in the DMG apps, so Finder/Spotlight display a generic icon after repair.

Verification

• cargo fmt --all -- --check
• targeted regression test passes: cargo test -p codex-plus-core --test installers macos_bundle_metadata_contains_silent_and_manager_apps -- --exact
• workflow YAML parses successfully
• git diff --check passes

Existing unrelated test failures

The full installers test binary currently has two pre-existing macOS companion-path assertion failures involving CodexPlusPlusManager vs codex-plus-plus-manager; this PR does not touch that path logic.

[BigPizzaV3]

复审确认修复方向合理：当前主线的修复入口仍引用不存在的 PNG，而 release 包实际提供 codex-plus-plus.icns；从当前 App bundle 的 Resources 复制 ICNS 并让 plist 指向它可以解决修复后图标丢失。当前分支与最新 main 冲突，请先 rebase/同步主线并重新跑安装器测试和三平台 CI；不要直接合并旧分支。

[BigPizzaV3]

修复方向仍成立：修复入口应复制并引用 bundle 中实际存在的 ICNS，而不是不存在的 PNG。请 rebase 当前 main，避免带入旧安装器命名断言，并重新运行 macOS installer 定向测试和三平台构建；当前冲突状态不合并。