fix: un-used variables and add sanity checks

[zsystm]

Description

applied feedbacks from @jeonghoyeo7 's code-reveiw

• DoLiquidStake must returns total new shares and total ls token mint amount because user can liquid stake multiple chunks.
• add sanity checks
  • use sdk.Int instead of int64 when calc chunksToLiquidUnstake to avoid un-expected overflow error.
  • avoid division by zero

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