feat(statetest-types): add BPO2ToAmsterdamAtTime15k fork spec

[rakita]

Summary

• Add new BPO2ToAmsterdamAtTime15k variant to the ForkSpec enum for handling the BPO2 to Amsterdam transition at time 15k

Test plan

• Verify the code compiles successfully
• Confirm the new variant is properly positioned in the enum ordering

[codspeed-hq]

Merging this PR will not alter performance

✅ 173 untouched benchmarks

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_{Comparing feat/add-bpo2-to-amsterdam-fork-spec (7179746) with main (9a5c214)}