fix(api): use params tipset-key to load the tipset for gas premium calculation

[akaladarshi]

Proposed Changes

• Use the TipSetKey if it is provided in the API params instead of getting heaviest tipset (only use heaviest if API params doesn't have a TipSetKey)

Additional Info

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[Copilot]

Pull Request Overview

This PR fixes the gas premium calculation in the Filecoin API by using the provided tipset-key parameter instead of always defaulting to the heaviest tipset.

• Updated GasEstimateFeeCap and GasEstimateGasPremium functions to accept and use the TipSetKey parameter
• Modified API layer to pass the tsk parameter through to the underlying utility functions
• Changed parameter names from ignored (_) to used (tsk/ts) to reflect actual usage

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
node/impl/gasutils/gasutils.go	Updated core gas estimation functions to accept and use TipSetKey parameter instead of hardcoded heaviest tipset
node/impl/full/gas.go	Modified API layer functions to pass through the tipset key parameter and use it in gas estimation calls

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