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_xsd_generated.go

@@ -623,13 +623,16 @@ type CopyObjectResult struct {
 	ETag         string    `xml:"ETag"`
 }
 
-func (t *CopyObjectResult) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
+// MarshalXML has a value receiver: handlers pass CopyObjectResult by value, and
+// encoding/xml does not call a pointer-receiver MarshalXML on a non-addressable
+// value, which silently fell back to time.Time's RFC 3339 encoding.
+func (t CopyObjectResult) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
 	type T CopyObjectResult
 	var layout struct {
 		*T
 		LastModified *xsdDateTime `xml:"LastModified"`
 	}
-	layout.T = (*T)(t)
+	layout.T = (*T)(&t)
 	layout.LastModified = (*xsdDateTime)(&layout.T.LastModified)
 	return e.EncodeElement(layout, start)
 }
@@ -1512,8 +1515,15 @@ type xsdDateTime time.Time
 func (t *xsdDateTime) UnmarshalText(text []byte) error {
 	return _unmarshalTime(text, (*time.Time)(t), "2006-01-02T15:04:05.999999999")
 }
+
+// s3TimestampFormat is the timestamp layout AWS S3 uses in XML responses:
+// UTC with exactly three fractional digits. Trimming trailing zeros (".56Z",
+// or no fraction at all) breaks clients that parse with a fixed-width pattern,
+// e.g. minio-java's "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'".
+const s3TimestampFormat = "2006-01-02T15:04:05.000Z"
+
 func (t xsdDateTime) MarshalText() ([]byte, error) {
-	return _marshalTime((time.Time)(t), "2006-01-02T15:04:05.999999999")
+	return []byte((time.Time)(t).UTC().Format(s3TimestampFormat)), nil
 }
 func (t xsdDateTime) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
 	if (time.Time)(t).IsZero() {