Fix MemoryStreamArchiveReader.GetStream() failing in some cases

[bdach]

MemoryStreamArchiveReader introduced in 0657b55 would previously use MemoryStream.GetBuffer() to retrieve the underlying byte buffer with stream data. However, this is not generally the method you would want, for two reasons:

1. It can fail if the stream wasn't created in the way that supports it.
2. As per docs, it will return the raw contents of the buffer, including potentially unused bytes.

To fix, use MemoryStream.ToArray() instead, which avoids both pitfalls. Notably, ToArray() always returns the full contents of the buffer, regardless of Position, as per documentation.

[peppy]

Was this failing somewhere? The change was mostly intentional as it avoids the copy. The Length spec should avoid including unused bytes, as far as I can tell.

[bdach]

Yes, when working on #24450 this was causing failures in tests. Not every MemoryStream allows accessing the internal buffer. For instance, if the following path is taken in ImportTask:

osu/osu.Game/Database/ImportTask.cs

Lines 61 to 66 in f3b6aa5

	if (Stream is not MemoryStream memoryStream)
	{
	// Path used primarily in tests (converting `ManifestResourceStream`s to `MemoryStream`s).
	memoryStream = new MemoryStream(Stream.ReadAllBytesToArray());
	Stream.Dispose();
	}

which will happen primarily in tests, you cannot call GetBuffer() on that stream, as it will throw.

[peppy]

That is an absolutely stupid API. There also seems to be no way of specifying the publicly exposed part apart from using the full public MemoryStream(byte[] buffer, int index, int count, bool writable, bool publiclyVisible).