[Mono.AndroidTools] Enable nullable reference types in AdbSyncNotification.cs

[Copilot]

AdbSyncNotification.cs ships in Mono.AndroidTools.dll but compiled in a nullable-oblivious context. This opts the file into NRT, making its existing null contract explicit: LocalPath is null for directory/remove/preserve operations, while RemotePath is always non-null.

Changes

• src/Mono.AndroidTools/Adb/AdbSyncNotification.cs
  • Added #nullable enable as the first line.
  • Annotated LocalPath property and the localPath constructor parameter as string?; RemotePath stays non-nullable.

public AdbSyncNotification (AdbSyncKind kind, string remotePath, string? localPath, long size)
...
public string? LocalPath { get; private set; }

No null-throwing is required (netstandard2.0, so no ArgumentNullException.ThrowIfNull), and callers in AdbSyncContext.cs, AdbSyncClient.cs, and AndroidDevice.cs are not nullable-enabled, so no call-site changes or warnings result.

[Copilot]

Pull request overview

This PR opts AdbSyncNotification.cs (shipped in Mono.AndroidTools.dll) into nullable reference types (NRT) to make its nullability contract explicit: LocalPath can be null for non-file operations, while RemotePath is intended to always be non-null.

Changes:

• Enabled NRT for AdbSyncNotification.cs via #nullable enable.
• Annotated LocalPath and the corresponding constructor parameter as string? while keeping RemotePath as non-nullable.

Comments suppressed due to low confidence (1)

src/Mono.AndroidTools/Adb/AdbSyncNotification.cs:43

• RemotePath is declared non-nullable, but the constructor currently accepts remotePath without validating it. Since this is a public API and most callers are in nullable-oblivious contexts, null can still be passed at runtime, breaking the non-null contract and potentially causing later NullReferenceExceptions. Add an ArgumentNullException guard to keep the contract sound.

		public AdbSyncNotification (AdbSyncKind kind, string remotePath, string? localPath, long size)
		{
			this.Kind = kind;
			this.RemotePath = remotePath;
			this.LocalPath = localPath;
			this.Size = size;
		}