Remove macro-based header guard in favor of pragma once

[Copilot]

The REF_COLLOCATION_DATA_DEFINED macro created unnecessary coupling between collocation_common.hpp and hdf5_test_serialization_impl.hpp, enforcing include ordering via compile-time error.

Changes

• Removed #define REF_COLLOCATION_DATA_DEFINED from tests/collocation_common.hpp
• Removed #ifndef REF_COLLOCATION_DATA_DEFINED check from tests/hdf5_test_serialization_impl.hpp
• Added direct include of collocation_common.hpp in tests/hdf5_test_serialization_impl.hpp to make it self-contained

Both files use #pragma once for include guards, which properly handles multiple inclusions. The macro was redundant and created fragile ordering dependencies. The header is now self-contained and doesn't require consumers to include dependencies in a specific order.

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