[BUG] BlobOpenWriteOptions.BufferSize ignored in BlockBlobClient.OpenWriteAsync

[rlrossiter]

Library name and version

Azure.Storage.Blobs 12.23.0

Describe the bug

The documentation for [BlobOpenWriteOptions.BufferSize](https://github.com/Azure/azure-sdk-for-net/blob/710a44d062b41c3e8a59ba8676618e939d2ce0c2/sdk/storage/Azure.Storage.Blobs/src/Models/BlobOpenWriteOptions.cs#L18-L22) says the default buffer used is 4MB and the max is 4000MB. That bufferSize is passed to StorageWriteStream, where the actual buffer size is calculated to be [Math.Min(Constants.MB, bufferSize](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/storage/Azure.Storage.Common/src/Shared/StorageWriteStream.cs#L96).

This is because PooledMemoryStream ends up using an ArrayPool to back the buffer, which has a max array size of type int (and Shared in reality has a max of 1024 * 1024) so the Math.Min makes sense. It should just be caught in ValidateBufferSize() rather than silently changing what was requested.

Expected behavior

    private const long OneMB = 1024 * 1024;
    private static readonly BlobOpenWriteOptions Options = new BlobOpenWriteOptions
    {
        BufferSize = 1024 * OneMB,
    };

    static void Main(string[] args)
    {
        string BlobUri = $"https://sasenderscaletest0.blob.core.windows.net/fmstest/testblob";
        BlobClient blobClient = new BlobClient(new Uri(BlobUri), credential: new DefaultAzureCredential());
        using var stream = blobClient.OpenWriteAsync(true, options: Options).Result;
    }

Expected:
stream._buffer.MaxArraySize == stream._bufferSize == 1073741824

Actual behavior

stream._bufferSize == 1073741824
stream._buffer.MaxArraySize == 1048576

Reproduction Steps

internal class Program
{
    private const long OneMB = 1024 * 1024;
    private static readonly BlobOpenWriteOptions Options = new BlobOpenWriteOptions
    {
        BufferSize = 1024 * OneMB,
    };

    static void Main(string[] args)
    {
        string BlobUri = $"https://mysa.blob.core.windows.net/container/testblob";
        BlobClient blobClient = new BlobClient(new Uri(BlobUri), credential: new DefaultAzureCredential());
        using var stream = blobClient.OpenWriteAsync(true, options: Options).Result;
    }
}

Environment

No response

[github-actions]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @xgithubtriage.