s3api: always write XML timestamps with three fractional digits (#11520)

CopyObject responses carried LastModified values such as
"2026-09-29T20:30:04.56Z": trailing zeros of the fractional seconds were
trimmed, and a whole-second value had no fraction at all. AWS S3 always
writes exactly three digits ("...04.560Z"), and clients that parse with a
fixed-width pattern reject anything else. minio-java 8.6.0
(yyyy-MM-dd'T'HH:mm:ss.SSS'Z') throws DateTimeParseException, so roughly
one CopyObject in ten fails on the client even though the copy succeeded.

Two causes:

- xsdDateTime marshalled with "2006-01-02T15:04:05.999999999", which
  drops trailing zeros. It now writes UTC with ".000Z".
- CopyObjectResult.MarshalXML had a pointer receiver, but the handlers
  pass the result by value, so encoding/xml never called it and fell back
  to time.Time's RFC 3339 encoding. It now has a value receiver.
  CopyPartResult had no custom marshaller at all; it now uses xsdDateTime.

Follow-up to #8394 / #8398, which truncated these timestamps to
milliseconds but kept the trimmed format.

weed/s3api/s3api_xml_timestamp_test.go

@@ -0,0 +1,56 @@
+package s3api
+
+import (
+	"encoding/xml"
+	"strings"
+	"testing"
+	"time"
+)
+
+// S3 clients such as minio-java parse LastModified with a fixed-width
+// "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'" pattern, so trailing zeros in the
+// fractional seconds must not be trimmed.
+func TestXMLTimestampsHaveFixedMilliseconds(t *testing.T) {
+	cases := []struct {
+		in   time.Time
+		want string
+	}{
+		{time.Date(2026, 9, 29, 20, 30, 4, 560_000_000, time.UTC), "2026-09-29T20:30:04.560Z"},
+		{time.Date(2026, 9, 29, 20, 30, 4, 500_000_000, time.UTC), "2026-09-29T20:30:04.500Z"},
+		{time.Date(2026, 9, 29, 20, 30, 4, 0, time.UTC), "2026-09-29T20:30:04.000Z"},
+		{time.Date(2026, 9, 29, 20, 30, 4, 123_456_789, time.UTC), "2026-09-29T20:30:04.123Z"},
+		{time.Date(2026, 9, 29, 14, 30, 4, 560_000_000, time.FixedZone("MDT", -6*3600)), "2026-09-29T20:30:04.560Z"},
+	}
+	for _, c := range cases {
+		for name, v := range map[string]any{
+			// by value, the way the handlers pass them to writeSuccessResponseXML
+			"CopyObjectResult":       CopyObjectResult{ETag: "e", LastModified: c.in},
+			"CopyPartResult":         CopyPartResult{ETag: "e", LastModified: c.in},
+			"CopyObjectResult (ptr)": &CopyObjectResult{ETag: "e", LastModified: c.in},
+			"CopyPartResult (ptr)":   &CopyPartResult{ETag: "e", LastModified: c.in},
+		} {
+			out, err := xml.Marshal(v)
+			if err != nil {
+				t.Fatalf("%s: %v", name, err)
+			}
+			if !strings.Contains(string(out), "<LastModified>"+c.want+"</LastModified>") {
+				t.Errorf("%s(%v): got %s, want LastModified %s", name, c.in, out, c.want)
+			}
+		}
+	}
+}
+
+func TestXMLTimestampRoundTrip(t *testing.T) {
+	in := CopyObjectResult{ETag: "e", LastModified: time.Date(2026, 9, 29, 20, 30, 4, 560_000_000, time.UTC)}
+	out, err := xml.Marshal(&in)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var back CopyObjectResult
+	if err := xml.Unmarshal(out, &back); err != nil {
+		t.Fatal(err)
+	}
+	if !back.LastModified.Equal(in.LastModified) {
+		t.Fatalf("round trip: got %v, want %v", back.LastModified, in.LastModified)
+	}
+}