Prevent overwriting matched prefix cache blocks with recomputed KV

[santhnm2]

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What does this PR do?

When a prefill chunk recomputes tokens that fall inside a hash-matched KV block, redirects those writes to the dummy block instead of the shared real one.

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[santhnm2]

/ok to test 2e3faff

[santhnm2]

/ok to test bfbaa8f

[nemo-automation-bot]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/33002432584