Fix Preview Output Corruption for Custom PreviewWriter

Always wrap PreviewWriter with newlineWriter to ensure newlines.

When a custom PreviewWriter is provided (like in the web app),
preview output wasn't wrapped with newlineWriter, causing JSON
records to be concatenated without newlines between them. This
was especially noticeable with single records.

Author: cwarden

dryrun_test.go

@@ -0,0 +1,96 @@
+package batch
+
+import (
+	"bytes"
+	"context"
+	"strings"
+	"testing"
+
+	force "github.com/ForceCLI/force/lib"
+)
+
+func TestDryRunAddsNewlinesBetweenRecords(t *testing.T) {
+	// Create a buffer to capture output
+	var buf bytes.Buffer
+
+	// Create an execution with a custom PreviewWriter
+	exec := &Execution{
+		PreviewWriter: &buf,
+	}
+
+	// Create a channel with test records
+	records := make(chan force.ForceRecord, 2)
+	records <- force.ForceRecord{"Id": "001000000000001", "Name": "Test Account 1"}
+	records <- force.ForceRecord{"Id": "001000000000002", "Name": "Test Account 2"}
+	close(records)
+
+	// Run dry run
+	ctx := context.Background()
+	_, err := exec.dryRun(ctx, records)
+	if err != nil {
+		t.Fatalf("dryRun returned error: %v", err)
+	}
+
+	// Check output
+	output := buf.String()
+	lines := strings.Split(strings.TrimSpace(output), "\n")
+
+	// Should have exactly 2 lines
+	if len(lines) != 2 {
+		t.Errorf("Expected 2 lines, got %d. Output:\n%s", len(lines), output)
+	}
+
+	// Each line should be valid JSON
+	expectedLines := []string{
+		`{"Id":"001000000000001","Name":"Test Account 1"}`,
+		`{"Id":"001000000000002","Name":"Test Account 2"}`,
+	}
+
+	for i, line := range lines {
+		if line != expectedLines[i] {
+			t.Errorf("Line %d mismatch:\nExpected: %s\nGot:      %s", i+1, expectedLines[i], line)
+		}
+	}
+}
+
+func TestDryRunWithSingleRecord(t *testing.T) {
+	// Create a buffer to capture output
+	var buf bytes.Buffer
+
+	// Create an execution with a custom PreviewWriter
+	exec := &Execution{
+		PreviewWriter: &buf,
+	}
+
+	// Create a channel with a single test record
+	records := make(chan force.ForceRecord, 1)
+	records <- force.ForceRecord{"Id": "001000000000001", "Name": "Test Account"}
+	close(records)
+
+	// Run dry run
+	ctx := context.Background()
+	_, err := exec.dryRun(ctx, records)
+	if err != nil {
+		t.Fatalf("dryRun returned error: %v", err)
+	}
+
+	// Check output
+	output := buf.String()
+
+	// Should end with a newline
+	if !strings.HasSuffix(output, "\n") {
+		t.Errorf("Output should end with a newline. Got: %q", output)
+	}
+
+	// Should have exactly one line when trimmed
+	trimmed := strings.TrimSpace(output)
+	if strings.Contains(trimmed, "\n") {
+		t.Errorf("Output should contain only one record. Got: %q", output)
+	}
+
+	// Should be valid JSON
+	expected := `{"Id":"001000000000001","Name":"Test Account"}`
+	if trimmed != expected {
+		t.Errorf("Output mismatch:\nExpected: %s\nGot:      %s", expected, trimmed)
+	}
+}

execution.go

@@ -303,6 +303,8 @@ func (e *Execution) dryRun(ctx context.Context, records <-chan force.ForceRecord
 	previewWriter := e.PreviewWriter
 	if previewWriter == nil {
 		previewWriter = newlineWriter{w: os.Stdout}
+	} else {
+		previewWriter = newlineWriter{w: previewWriter}
 	}
 
 	waitForRecord := 5 * time.Second