Remove null-forgiving operator (!) in BaseTest.AssertApkSigned

[Copilot]

Process.Start can return null, but the previous code used proc! to suppress the compiler warning, risking a raw NullReferenceException on failure. Replace with an explicit ?? throw to surface a clear error.

Changes

• BaseTest.cs: Replace proc!.StandardOutput with a null-coalescing throw on the Process.Start result:

// Before
using var proc = Process.Start (psi);
string stdout = proc!.StandardOutput.ReadToEnd ();

// After
using var proc = Process.Start (psi) ?? throw new InvalidOperationException ($"Failed to start '{apksignerExe}'.");
string stdout = proc.StandardOutput.ReadToEnd ();

Subsequent dereferences of proc (StandardError, WaitForExit, ExitCode) are already !-free and require no change.

[Copilot]

Pull request overview

This PR hardens the BaseTest.AssertApkIsSigned test helper by removing a null-forgiving dereference on the Process.Start(ProcessStartInfo) result and replacing it with an explicit null check that throws a clear exception when the process fails to start.

Changes:

• Replace proc!.StandardOutput... with Process.Start (psi) ?? throw ... to avoid a potential NullReferenceException.
• Keep the rest of the process interaction (StandardError, WaitForExit, ExitCode) unchanged and !-free.