MultiplayerSynchronizer - SetVisibilityFor spam errors if Peer is already removed.

### Tested versions

v4.5.beta5.mono.official [c81fd6c51]

### System information

Godot v4.5.beta5.mono - Windows 10 (build 19045) - Multi-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3060 Ti (NVIDIA; 32.0.15.6094) - AMD Ryzen 7 3700X 8-Core Processor (16 threads) - 31.92 GiB memory

### Issue description

I use BodyEntered and BodyExited to update my MultiplayerSynchronizer with SetVisibilityFor, this areas is attached to each Player in my game, however when this player disconnects with

```Csharp
Multiplayer.MultiplayerPeer.DisconnectPeer((int)id);
```

How I'm supposed to clean the Visible peers after they disconnect without get this amount of spam?

### Steps to reproduce

Just call SetVisibilityFor((int)_peerId, false) to any peer after it disconnects.

### Minimal reproduction project (MRP)

[network-test.zip](https://github.com/user-attachments/files/22258996/network-test.zip)

Start with MultiplayerSynchronizer and the SetVisibilityFor entry point, then trace how peer removal from DisconnectPeer interacts with the BodyEntered and BodyExited callbacks. Use the linked network-test.zip reproduction to trigger SetVisibilityFor for a disconnected peer; done means the disconnect no longer produces repeated errors while visibility updates remain correct.

Written by the indexing model from the issue text.