Insert the cells from the start position

[dhruvmanila]

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
fixes: astral-sh/ruff-vscode#573

Test Plan

Consider the following three cells:

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

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

def hello():
    print("hello world")

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",
        },
    },
]

[dhruvmanila]

TODO: I'm going to write a test case for this.

Edit: Added a test case for this scenario.

[github-actions]

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