Summary

• Add test infrastructure for DTLS socket testing
• Implement 20 unit tests for dtls_socket class
• Achieve full test coverage for DTLS socket public interface

Changes

Test Infrastructure (tests/helpers/dtls_test_helpers.h)

• ssl_context_wrapper: RAII wrapper for SSL_CTX with automatic cleanup
• test_certificate_generator: Generate self-signed test certificates in memory
• dtls_context_factory: Create server/client DTLS contexts for testing
• find_available_udp_port: Port discovery utility for test isolation

Unit Tests (tests/unit/test_dtls_socket.cpp)

Construction Tests

• ConstructWithValidContext - Verify socket creation with valid SSL context
• ConstructWithNullContextThrows - Verify exception on null context
• InitialStateNotHandshakeComplete - Verify initial handshake state

Callback Registration Tests

• SetReceiveCallback - Test receive callback registration
• SetErrorCallback - Test error callback registration
• SetPeerEndpoint - Test peer endpoint configuration

Start/Stop Receive Tests

• StartReceiveDoesNotThrow - Verify safe start receive
• StopReceiveDoesNotThrow - Verify safe stop receive
• MultipleStartReceiveCalls - Test idempotent start
• MultipleStopReceiveCalls - Test idempotent stop

Handshake Tests

• ClientServerHandshake - Full client-server DTLS handshake
• SendBeforeHandshakeFails - Verify send fails before handshake

Thread Safety Tests

• ConcurrentCallbackRegistration - Test thread-safe callback registration
• ConcurrentEndpointAccess - Test thread-safe endpoint access

Integration Tests

• SendReceiveAfterHandshake - Data exchange after handshake
• BidirectionalCommunication - Two-way data exchange
• LargePayload - Test with 1000-byte payloads

Error Handling Tests

• ErrorCallbackInvokedOnSocketClose - Verify error callback on socket close
• SocketAccessReturnsValidSocket - Verify socket accessor
• DestructorAfterStartReceive - Test cleanup after async operations

Test Results

All 20 tests pass:

[==========] Running 20 tests from 2 test suites.
[  PASSED  ] 20 tests.

Test plan

• All unit tests pass locally
• Build passes with TLS support enabled
• No memory leaks (RAII patterns used throughout)
• Thread safety tests pass

Closes #401