What

Add dedicated unit tests for 5 remaining untested QUIC protocol modules. Part of #953.

Scope

Why

• QUIC crypto and connection are foundational protocol components — untested code risks silent TLS/handshake failures
• PMTUD (Path MTU Discovery) controller affects packet sizing on all QUIC connections
• Part of epic Expand unit test coverage from 40% to 80% #953 to raise coverage from ~48% to 80%

Where

• Source: src/protocols/quic/, src/internal/
• Tests: tests/unit/ (new test files)
• CMake: tests/CMakeLists.txt (register new tests)

How

Technical Approach

1. Create unit test files following existing project patterns (see quic_*_test.cpp files)
2. crypto: test key derivation, AEAD encryption/decryption, key update
3. connection: test state machine transitions, handshake flow, packet processing
4. pmtud_controller: test MTU probing, path validation, size selection
5. quic_socket: test socket creation, binding, send/receive

Acceptance Criteria

• All 5 modules have dedicated unit test files
• Tests pass on local build and CI (Ubuntu, macOS)
• No GTEST_SKIP() markers
• Tests cover construction, public methods, and error paths