AudioStreamPlaybackPolyphonic doesn't emit `finished` signal for each AudioStream neither allow get the reference from ID

### Tested versions

Godot 4.2.1

### System information

Godot v4.2.1.stable - macOS 13.4.1 - Vulkan (Forward+) - integrated Apple M1 - Apple M1 (8 Threads)

### Issue description

`AudioStreamPlayer`s don't emit the `finished` signal after playing a stream from an Audio Playback track.

### Steps to reproduce

Add an animation with an Audio Playback track. Connect the `AudioStreamPlayer`'s `finished` signal to a method in a script, and do a `print()`

### Minimal reproduction project (MRP)

N/A

Start with the AudioStreamPlaybackPolyphonic implementation and the animation Audio Playback track described in the reproduction steps. Trace how each played AudioStream is tracked and how the AudioStreamPlayer finished signal is emitted; done means each stream emits finished and its reference can be retrieved from its ID.

Written by the indexing model from the issue text.