## Summary

The cause of this bug is from
#12575 which was itself a bug fix
but the fix wasn't completely correct.

fixes: #14768 
fixes: astral-sh/ruff-vscode#644

## Test Plan

Consider the following three cells:

1.
```python
class Foo:
    def __init__(self):
        self.x = 1

    def __str__(self):
        return f"Foo({self.x})"
```

2.
```python
def hello():
    print("hello world")
```

3.
```python
y = 1
```

The test case is moving cell 2 to the top i.e., cell 2 goes to position
1 and cell 1 goes to position 2.

Before this fix, it can be seen that the cells were pushed at the end of
the vector:

```
  12.643269917s  INFO ruff:main ruff_server::edit::notebook: Before update: [
    NotebookCell {
        document: TextDocument {
            contents: "class Foo:\n    def __init__(self):\n        self.x = 1\n\n    def __str__(self):\n        return f\"Foo({self.x})\"",
        },
    },
    NotebookCell {
        document: TextDocument {
            contents: "def hello():\n    print(\"hello world\")",
        },
    },
    NotebookCell {
        document: TextDocument {
            contents: "y = 1",
        },
    },
]
  12.643777667s  INFO ruff:main ruff_server::edit::notebook: After update: [
    NotebookCell {
        document: TextDocument {
            contents: "y = 1",
        },
    },
    NotebookCell {
        document: TextDocument {
            contents: "class Foo:\n    def __init__(self):\n        self.x = 1\n\n    def __str__(self):\n        return f\"Foo({self.x})\"",
        },
    },
    NotebookCell {
        document: TextDocument {
            contents: "def hello():\n    print(\"hello world\")",
        },
    },
]
```

After the fix in this PR, it can be seen that the cells are being pushed
at the correct `start` index:

```
   6.520570917s  INFO ruff:main ruff_server::edit::notebook: Before update: [
    NotebookCell {
        document: TextDocument {
            contents: "class Foo:\n    def __init__(self):\n        self.x = 1\n\n    def __str__(self):\n        return f\"Foo({self.x})\"",
        },
    },
    NotebookCell {
        document: TextDocument {
            contents: "def hello():\n    print(\"hello world\")",
        },
    },
    NotebookCell {
        document: TextDocument {
            contents: "y = 1",
        },
    },
]
   6.521084792s  INFO ruff:main ruff_server::edit::notebook: After update: [
    NotebookCell {
        document: TextDocument {
            contents: "def hello():\n    print(\"hello world\")",
        },
    },
    NotebookCell {
        document: TextDocument {
            contents: "class Foo:\n    def __init__(self):\n        self.x = 1\n\n    def __str__(self):\n        return f\"Foo({self.x})\"",
        },
    },
    NotebookCell {
        document: TextDocument {
            contents: "y = 1",
        },
    },
]
```