detect: cache Default() result with sync.Once, add Reset()

Co-authored-by: taigrr <8261498+taigrr@users.noreply.github.com>

detect/detect_test.go

@@ -20,3 +20,26 @@ func TestDefaultDoesNotPanic(t *testing.T) {
 	// May return error if no managers available; that's fine.
 	_, _ = Default()
 }
+
+func TestDefaultCachesResult(t *testing.T) {
+	Reset()
+	m1, err1 := Default()
+	m2, err2 := Default()
+	if m1 != m2 {
+		t.Error("expected Default() to return the same Manager on repeated calls")
+	}
+	if err1 != err2 {
+		t.Error("expected Default() to return the same error on repeated calls")
+	}
+	// Exactly one of m1/err1 must be non-nil (either a manager was found or not).
+	if (m1 == nil) == (err1 == nil) {
+		t.Error("expected exactly one of Manager or error to be non-nil")
+	}
+}
+
+func TestResetAllowsRedetection(t *testing.T) {
+	_, _ = Default()
+	Reset()
+	// After reset, defaultOnce should be fresh; calling Default() again should work.
+	_, _ = Default()
+}