What

Add dedicated unit tests for 6 untested infrastructure and service modules. Part of #953.

Scope

Note: grpc_official_wrapper excluded — requires optional gRPC dependency (BUILD_GRPC_OFFICIAL=ON).

Why

• client/server are the core base classes for all messaging implementations
• health_monitor and service_registry provide runtime monitoring/discovery
• memory_profiler tracks allocation patterns for performance analysis
• Part of epic Expand unit test coverage from 40% to 80% #953 to raise coverage from ~48% to 80%

Where

• Source: src/core/, src/experimental/
• Tests: tests/unit/ (new test files)
• CMake: tests/CMakeLists.txt (register new tests)

How

Technical Approach

1. client/server: test lifecycle (create, start, stop), configuration, state transitions
2. health_monitor: test health check registration, status reporting, threshold alerts
3. service_registry: test service registration, discovery, deregistration
4. memory_profiler: test allocation tracking, reporting, memory stats
5. reliable_udp_client: test reliability layer, retransmission, ordering

Acceptance Criteria

• All 6 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