[AI] Fix NPE in PreSharedKeyExtensionMessage for TLS 1.3 PSK handshake

[ic0ns]

Summary

• Fixed NullPointerException in PreSharedKeyExtensionMessage.copyPskSets method
• Added missing initialization of identities and binders lists in Config constructor
• Added comprehensive unit tests to verify the fix

Description

The issue reported in #188 was caused by the copyPskSets method in PreSharedKeyExtensionMessage not setting the identity field in PSKIdentity objects. This caused a NullPointerException when PreSharedKeyExtensionHandler tried to access pskIdentity.getIdentity().getValue() during TLS 1.3 PSK handshakes.

The fix adds the missing line:

pskIdentity.setIdentity(pskSets.get(x).getPreSharedKeyIdentity());

Additionally, I found and fixed another issue where the identities and binders lists were not initialized in the Config constructor when no PSK sets were configured, which could lead to NPEs in other scenarios.

Test plan

• Added unit tests for PreSharedKeyExtensionMessage covering:
  • PSK sets copied correctly with identity field set
  • Limit PSKs to one functionality
  • Empty PSK sets configuration
• All existing tests pass
• Code compiles successfully
• Spotless formatting applied

Fixes #188