bug: opcode 0x08 SIGNEXTEND not conform to spec

[enitrat]

Bug Report

Kakarot version:
last version

Current behavior:
The current implementation is

    // @notice 0x0B - SIGNEXTEND
    // @dev Exp operation
    // @custom:since Frontier
    // @custom:group Stop and Arithmetic Operations
    // @custom:gas 5
    // @custom:stack_consumed_elements 2
    // @custom:stack_produced_elements 1
    // @param ctx The pointer to the execution context.
    // @return ExecutionContext The pointer to the execution context.
    func exec_signextend{
        syscall_ptr: felt*,
        pedersen_ptr: HashBuiltin*,
        range_check_ptr,
        bitwise_ptr: BitwiseBuiltin*,
    }(ctx: model.ExecutionContext*) -> model.ExecutionContext* {
        alloc_locals;

        // Stack input:
        // 0 - a: number.
        // 1 - b: exponent.
        let stack = ctx.stack;
        let (stack, popped) = Stack.pop_n(self=stack, n=2);
        let b = popped[0];
        let a = popped[1];

        // Value is already a uint256
        let stack: model.Stack* = Stack.push(self=stack, element=a);
        let ctx = apply_context_changes(ctx=ctx, stack=stack, gas_cost=GAS_COST_SIGNEXTEND);
        return ctx;
    }

https://github.com/kkrt-labs/kakarot/blob/main/src/kakarot/instructions/stop_and_arithmetic_operations.cairo#L414-L443

Expected behavior:
The expected behavior is the one explained in the whitepaper, further explained in this post https://ethereum.stackexchange.com/questions/63062/evm-signextend-opcode-explanation

For example, executing the following opcodes

// Example 1
PUSH1 0xFF
PUSH1 0
SIGNEXTEND

Should result in 0xffff....ffff (u256_max -1)

Steps to reproduce:

Related code:

Other information: