test(tax2gas): add test for post handler

[TropicalDog17]

Summary of changes

• Remove ctx.isCheckTx() in posthandler

• Ensure non-negative coin amounts in fee deduction to prevent panic and better error message.

    // Deduct feeCoins with paid amount  
    if paidAmount.Ceil().RoundInt().Int64() > feeCoins.AmountOf(paidDenom).Int64() {  
       return ctx, errorsmod.Wrapf(sdkerrors.ErrInsufficientFee, "insufficient fee: %s", feeCoins)  
    }  
    feeCoins = feeCoins.Sub(sdk.NewCoin(paidDenom, paidAmount.Ceil().RoundInt()))  
}

Report of required housekeeping

• Github issue OR spec proposal link
• Wrote tests
• Updated API documentation (client/lcd/swagger-ui/swagger.yaml)
• Added a relevant changelog entry: clog add [section] [stanza] [message]

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(FOR ADMIN) Before merging

• Added appropriate labels to PR
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)
• Confirm added tests are consistent with the intended behavior of changes
• Ensure all tests pass