fix: EthGetBalance: lookup balance at correct state

[arajasek]

Related Issues

@raulk observed that some tooling was taken longer than expected to update balance. This is because the current EthBalance implementation uses the parent state of the input tipset, which is inconsistent with the Tipset <-> blockParam mapping we have.

Proposed Changes

• Use the TipsetState of the input param
  • I considered extracting this into some helper, but there isn't any other use case of it (nor is it needed anywhere else in the EthAPI), and I think it'll confuse usage with the existing LoadActor methods.
• Add a test

Additional Info

Checklist

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[arajasek]

This assertion fails without the change in this PR.