Close Go/.NET shell-test teardown gap

The "should kill shell process" e2e test exists in all five SDKs, but only
Node, Python and Rust destroy the session at the end of it. Go tears the whole
client down with client.ForceStop() and .NET simply ended at the last assert,
so neither ever exercised session.destroy after a session.shell.exec.

That gap is why Go and .NET stayed green on CLI 1.0.76-0 while Node, Python and
Rust hang: the hang is in session.destroy after a shell exec, and the two green
SDKs never made the call. Adding the teardown makes all five cover the same
sequence.

Verified locally against the same CLI binary (1.0.76-0, sha256 b8bfad2c...):
with the added Disconnect(), the Go test hangs past a 3-minute timeout where it
previously passed in seconds. The same test passes on CLI 1.0.73.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: e615d062-bcb7-431e-aa9c-d3e47405723a

Author: stephentoub

dotnet/test/E2E/RpcShellAndFleetE2ETests.cs

@@ -41,6 +41,8 @@ public async Task Should_Kill_Shell_Process()
         var killResult = await session.Rpc.Shell.KillAsync(execResult.ProcessId);
 
         Assert.True(killResult.Killed);
+
+        await session.DisposeAsync();
     }
 
     [Fact]

go/internal/e2e/rpc_shell_and_fleet_e2e_test.go

@@ -81,6 +81,10 @@ func TestRPCShellAndFleetE2E(t *testing.T) {
 		if !kill.Killed {
 			t.Errorf("Expected shell.kill to report Killed=true, got %+v", kill)
 		}
+
+		if err := session.Disconnect(); err != nil {
+			t.Fatalf("Failed to disconnect session: %v", err)
+		}
 	})
 
 	t.Run("should start fleet and complete custom tool task", func(t *testing.T) {