Use structured error types for fee denom validation in CreateChainletStack

[Copilot]

The error handling for unsupported fee denominations was inconsistent between CreateChainletStack (using fmt.Errorf) and updateChainletStackFees (using cosmossdkerrors.Wrapf with types.ErrInvalidDenom).

Changes

• Added cosmossdkerrors import to msg_server_create_chainlet_stack.go
• Replaced fmt.Errorf with cosmossdkerrors.Wrapf(types.ErrInvalidDenom, ...) for denom validation errors

This enables programmatic error type checking and aligns with the existing pattern:

// Before
if !ok {
    return nil, fmt.Errorf("denom %s not supported for escrow deposits, supported %v", msg.Fees.Denom, denoms)
}

// After
if !ok {
    return nil, cosmossdkerrors.Wrapf(
        types.ErrInvalidDenom,
        "denom %s not supported for escrow deposits",
        msg.Fees.Denom,
    )
}

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