Return the length of the playing stream for AudioStreamRandomizer

[JulianHeuser]

Fixes #105929
Fixes #104684

[Mickeon]

We should probably question what get_length() should generally return for AudioStreamRandomizer. (see #105954 💦). This looks ideal specifically for AudioStreamInteractive, but it may be entirely unexpected when fetching the length without intending to play the AudioStreamRandomizer first.

[JulianHeuser]

Either way, can you actually confirm this fixes #104684 ?

Yes, I've tested it and it plays everything in sequence as expected, so it's definitely caused by the same issue.

I see the problem though - It would be a bit unclear if get_length changes its value based on the state of the stream, but it also doesn't make sense for it to be static on streams that can have variable length. The same issue is present for AudioStreamInteractive. I think for consistency, it makes more sense to return the length of the currently playing stream rather than the max length, since there are a lot of cases where users will need to use streams of variable length with AudioStreamInteractive. If someone wants to get the max length of all streams, they can do that in code, or we can make a helper function for the types of streams that could use it.