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.