Here's the complete code, including the Fibonacci class with the `GetClosestFibonacciNumbers` method and the unit tests:

```c#
using System;
using Xunit;
using Shouldly;

public static class Fibonacci
{
    public static (int closest, int next) GetClosestFibonacciNumbers(double number)
    {
        if (number < 0)
        {
            return (0, 1);
        }

        int previous = 0;
        int current = 1;

        while (true)
        {
            int nextFibonacci = previous + current;

            if (nextFibonacci > number)
            {
                return (current, nextFibonacci);
            }

            previous = current;
            current = nextFibonacci;
        }
    }
}

public class FibonacciTests
{
    [Fact]
    public void TestClosestFibonacciNumbersForNegativeNumber()
    {
        // Arrange
        double number = -10.5;

        // Act
        (int closest, int next) = Fibonacci.GetClosestFibonacciNumbers(number);

        // Assert
        closest.ShouldBe(0);
        next.ShouldBe(1);
    }

    [Fact]
    public void TestClosestFibonacciNumbersForLargeNumber()
    {
        // Arrange
        double number = 1000000;

        // Act
        (int closest, int next) = Fibonacci.GetClosestFibonacciNumbers(number);

        // Assert
        closest.ShouldBe(832040);
        next.ShouldBe(1346269);
    }
}
```

Explanation:

1. The `Fibonacci` class is a static class containing the `GetClosestFibonacciNumbers` method.
2. The method takes a double `number` as input and returns a tuple of two integers, `closest` and `next`, representing the closest Fibonacci number and the next one.
3. If the input `number` is negative, the method returns (0, 1) because 0 is the closest Fibonacci number, and 1 is the next one.
4. If the input `number` is positive, the method calculates the Fibonacci sequence iteratively using a `while` loop.
5. Inside the loop, it calculates the next number in the sequence `nextFibonacci` by adding the two previous numbers, `previous` and `current`.
6. If `nextFibonacci` is greater than the input `number`, the method returns the tuple (current, nextFibonacci) as the closest and next Fibonacci numbers.
7. If not, the method updates `previous` and `current` for the next iteration.
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