Test NewPlainSinkSplit event routing behavior

Verify that ErrorEvent is routed to errOut and all other events go to
out, and that nil writer arguments fall back safely without panicking.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: anisaoshafi

internal/output/plain_sink_test.go

@@ -286,6 +286,32 @@ func TestPlainSink_TableWidth(t *testing.T) {
 	})
 }
 
+func TestPlainSinkSplit_RoutesErrorEventToErrOut(t *testing.T) {
+	var out, errOut bytes.Buffer
+	sink := NewPlainSinkSplit(&out, &errOut)
+
+	sink.Emit(ErrorEvent{Title: "Something failed"})
+
+	assert.Empty(t, out.String(), "ErrorEvent should not go to out")
+	assert.Contains(t, errOut.String(), "Something failed")
+}
+
+func TestPlainSinkSplit_RoutesOtherEventsToOut(t *testing.T) {
+	var out, errOut bytes.Buffer
+	sink := NewPlainSinkSplit(&out, &errOut)
+
+	sink.Emit(MessageEvent{Severity: SeverityInfo, Text: "hello"})
+
+	assert.Contains(t, out.String(), "hello")
+	assert.Empty(t, errOut.String(), "MessageEvent should not go to errOut")
+}
+
+func TestPlainSinkSplit_NilWritersFallback(t *testing.T) {
+	// nil writers should not panic (fallback to os.Stdout/os.Stderr)
+	sink := NewPlainSinkSplit(nil, nil)
+	assert.NotNil(t, sink)
+}
+
 func TestPlainSink_ErrReturnsNilOnSuccess(t *testing.T) {
 	var out bytes.Buffer
 	sink := NewPlainSink(&out)