fix: NullReferenceException in StopClient when StartHost fails

[Copilot]

When StartHost() fails during server setup (e.g., port already in use), the mode is set to Host but NetworkServer.localConnection remains null. Calling StopHost() then throws NullReferenceException when attempting to invoke OnServerDisconnect(NetworkServer.localConnection), preventing proper cleanup and leaving the NetworkManager in an inconsistent state.

Changes

• NetworkManager.StopClient(): Added null check before accessing localConnection

  // Before
  if (mode == NetworkManagerMode.Host)
      OnServerDisconnect(NetworkServer.localConnection);
  
  // After
  if (mode == NetworkManagerMode.Host && NetworkServer.localConnection != null)
      OnServerDisconnect(NetworkServer.localConnection);

• NetworkManagerTest.cs: Added tests for StopHost() and StopClient() behavior when localConnection is null

Behavior

After this change, calling StopHost() or StopClient() after a failed StartHost() will:

• Skip the OnServerDisconnect() call when localConnection is null
• Continue with normal disconnect flow via NetworkClient.Disconnect()
• Properly reset mode to Offline via OnClientDisconnectInternal()

Original prompt

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This section details on the original issue you should resolve

<issue_title>Null Ref in StopHost/Client if StartHost/Client failed before</issue_title>
<issue_description>Describe the bug
Calling the StopHost method (internally StopClient) results in a NullReferenceException (NetworkServer.localConnection is null) if it is called after the StartHost has previously failed.

In other words: It is not possible to reset the mode to offline if server or client creation fails (e.g. if the socket address, protocol, network address or connection is already in use (System.Net.Sockets.SocketException).

[IMPORTANT] How can we reproduce the issue, step by step:

1. Start as Host with instance A.
2. Start a second Host with instance B.
3. An exception (System.Net.Sockets.SocketException, code 0x80004005) will occur on instance B, which is correct.
4. The NetworkManager.singleton.mode of instance B is still host.
5. Call StopHost on instance B to set the mode to offline.
6. A NullReferenceException will occur because the NetworkServer.localConnection is null in the StopClient method that is called internally. This should be caught.
7. The mode on instance B remains Host instead of offline.

Expected behavior
After calling StopHost/Client, the NetworkManager.singleton.mode should be offline. Even if an exception occurs when calling.

Is there any other way to reset the mode to offline?

Desktop (please complete the following information):

• OS: Windows
• Build target: all
• Unity version: 2022.3.62f1
• Mirror branch: 96.0.1 from Unity Asset Store</issue_description>

Comments on the Issue (you are @copilot in this section)

@MrGadget1024 Sounds like a bug that we're not resetting that when StartHost fails as you describe. Will look into it when we get a chance.

• Fixes Null Ref in StopHost/Client if StartHost/Client failed before #4041

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