Refactor: reorganize C++ UT into domain-based subdirectories

[chenshengxin2026]

Summary

• Move existing C++ tests into domain-based subdirectories: hierarchical/, a2a3/, a5/, types/, hardware/
• Add shared stubs/test_stubs.cpp for reusable test infrastructure
• Introduce add_a2a3_runtime_test() CMake helper for runtime-linked tests
• Remove pto2_ prefix from all CMake function and variable names for consistency
• No new test coverage — structural reorganization only

New directory layout

tests/ut/cpp/
├── a2a3/              # A2A3 orchestration API tests
├── a5/                # A5 orchestration API tests
├── hardware/          # HCCL comm tests (requires_hardware label)
├── hierarchical/      # Orchestrator, ring, scheduler, scope, tensormap
├── stubs/             # Shared test stubs
├── types/             # Task interface type tests
└── CMakeLists.txt

Test plan

• C++ UT build succeeds: cmake -B build -S tests/ut/cpp && cmake --build build
• All 8 C++ tests pass: ctest --test-dir build --output-on-failure
• Python UT: 208 passed, 2 skipped

[gemini-code-assist]

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