refactor: replace runner xctestrun XML regex parsing with shared iOS XML helper

[thymikee]

Summary

src/platforms/ios/runner-xctestrun.ts still uses regex-based plist/XML extraction in the fallback path for reading:

• ProductPaths
• DependentProductPaths
• TestHostPath
• TestBundlePath
• UITargetAppPath

We recently added src/platforms/ios/xml.ts and already use it in the iOS perf path and plist fallback path. The remaining regex parsing in runner-xctestrun.ts should be moved to the same shared helper so we stop maintaining multiple XML/plist parsing approaches.

Why

• Reduce duplicated XML parsing logic across iOS platform code.
• Remove brittle regex parsing for plist/XML structures.
• Keep the shared XML handling centered in one small helper.
• Make future parsing fixes apply consistently across perf, plist, and xctestrun code paths.

Scope

• Replace extractPlistStringValues and extractPlistArrayStringValues in src/platforms/ios/runner-xctestrun.ts with traversal built on src/platforms/ios/xml.ts.
• Preserve the current best-effort fallback behavior.
• Keep the change scoped to the runner/xctestrun path.
• Add or update focused tests around fallback extraction behavior.

Notes

This is intentionally separate from PR #358 so the iOS device perf work stays focused.