[nullable] Enable nullable reference types in ItemWithID.cs

[Copilot]

ItemWithID.cs was compiling in a nullable-oblivious context. This opts it into #nullable enable with the minimal mechanical changes required.

Changes

• src/Xamarin.Installer.AndroidSDK/Xamarin.Installer.AndroidSDK/Common/ItemWithID.cs
  • Added #nullable enable after the license header, before using directives
  • Annotated Equals parameters to match their interface/base signatures:

    public bool Equals (ItemWithID? other)
    public override bool Equals (object? obj)

No runtime behavior changes. The existing if (other == null) guard already satisfies flow analysis; no ! operator needed.