refactor: Replace synchronized block with AtomicReference for DPoP nonce management

[pmathew92]

This PR improves the thread-safety implementation in the DPoP flow by replacing explicit synchronization with modern atomic operations.

Changes Made

1. DPoP Nonce Management (DPoP.kt)

• Replaced @Volatile var _auth0Nonce: String? with AtomicReference<String?>
• Removed synchronized block in storeNonce() method
• Updated all nonce read/write operations to use atomic .get() and .set() methods
• Updated test file to use AtomicReference API

2. KeyStore Property (DPoPUtil.kt)

• Removed unnecessary @Volatile annotation from keyStore property

Testing

• ✅ All existing unit tests pass (64 tasks completed successfully)
• ✅ Concurrent access test verified (storeNonce should handle concurrent access safely)
• ✅ No behavioral changes - purely an implementation refactor

This is a code quality improvement with no functional changes to the public API.