chore: update openiap apple version to 1.3.15 (#314)

## Summary

- Sync with OpenIAP apple 1.3.15
- Includes watchOS platform compatibility fixes
- iOS 15 compatibility for priceFormatStyle API

## Related

- OpenIAP PR: hyodotdev/openiap#81
- OpenIAP Release: https://cocoapods.org/pods/openiap

## Changes

- Update openiap-versions.json (apple: 1.3.14 → 1.3.15)

## Test Plan

- [x] `bun run lint:ci` passes
- [x] `bun run test` passes (221 tests)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
  * Updated Apple package version to 1.3.15.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: hyochan

openiap-versions.json

@@ -1,5 +1,5 @@
 {
-  "apple": "1.3.14",
+  "apple": "1.3.15",
   "google": "1.3.28",
   "gql": "1.3.17"
 }