Update agent tests for the ctx-cancel suspend contract

Rewrite the WithStopSignal-driven test in restore_test.go to use a
cancellable ctx. Update agent_test.go's "context cancellation"
case from asserting "cannot complete" failure to asserting a
SuspendedError. Add cancel_test.go covering both pre-first-turn
cancel (no LLM call, empty checkpoint persisted) and mid-run
cancel from inside a tool (just-completed turn preserved in the
checkpoint, second LLM call suppressed).

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/agent/agent_test.go

@@ -479,7 +479,7 @@ func TestRun(t *testing.T) {
 	)
 
 	t.Run(
-		"context cancellation",
+		"context cancellation triggers graceful suspend",
 		func(t *testing.T) {
 			t.Parallel()
 
@@ -500,8 +500,10 @@ func TestRun(t *testing.T) {
 
 			_, err := ag.Run(ctx, []llm.Message{userMessage("test")})
 
-			require.Error(t, err)
-			assert.Contains(t, err.Error(), "cannot complete")
+			var se *agent.SuspendedError
+			require.ErrorAs(t, err, &se)
+			require.NotNil(t, se.Checkpoint)
+			assert.Equal(t, agent.AgentStatusSuspended, se.Checkpoint.Status)
 			assert.Equal(t, 0, provider.calls)
 		},
 	)