Sure, here's an implementation of the GetClosestFibonacciNumbers method in C# that should satisfy the requirements and tests mentioned above:

public static class Fibonacci
{
public static (int closest, int next) GetClosestFibonacciNumbers(double number)
{
if (number <= 0)
{
return (0, 1);
}
    int fib1 = 0, fib2 = 1, fib3;
    while (fib2 < number)
    {
        fib3 = fib1 + fib2;
        fib1 = fib2;
        fib2 = fib3;
    }

    return (fib1, fib2);
}
}

This implementation first handles the case where the input number is less than or equal to 0 by returning (0, 1), which are the first two numbers in the Fibonacci sequence.

For positive input values, it iteratively calculates the Fibonacci numbers until it finds the largest number in the sequence that is smaller than or equal to the input number. It returns a tuple containing this closest Fibonacci number and the next one in the sequence.

This implementation should pass the two unit tests mentioned above.
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