fix(api): keep milestone due date when it is omitted in edit request (#39869)

`PATCH /repos/{owner}/{repo}/milestones/{id}` always overwrote the due
date with `due_on`, so a request that only changed other fields, such as
`{"state": "closed"}`, silently removed it.

The due date is now kept when `due_on` is omitted, and `"due_on": null`
or `"due_on": ""` remove it.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/common/deadline.go

@@ -6,8 +6,10 @@ package common
 import (
 	"time"
 
+	"gitea.dev/modules/optional"
 	"gitea.dev/modules/setting"
 	"gitea.dev/modules/timeutil"
+	"gitea.dev/modules/util"
 )
 
 func ParseDeadlineDateToEndOfDay(date string) (timeutil.TimeStamp, error) {
@@ -22,9 +24,33 @@ func ParseDeadlineDateToEndOfDay(date string) (timeutil.TimeStamp, error) {
 	return timeutil.TimeStamp(deadline.Unix()), nil
 }
 
-func ParseAPIDeadlineToEndOfDay(t *time.Time) timeutil.TimeStamp {
-	if t == nil || t.IsZero() || t.Unix() == 0 {
-		return 0
+func ApplyAPIDeadlineTimestamp(field *timeutil.TimeStamp, input optional.Option[*string], remove *bool) (changed bool, err error) {
+	old := *field
+	if remove != nil && *remove || input.Has() && (input.Value() == nil || *input.Value() == "") {
+		*field = 0
+		return old != *field, nil
 	}
-	return timeutil.TimeStamp(time.Date(t.Year(), t.Month(), t.Day(), 23, 59, 59, 0, setting.DefaultUILocation).Unix())
+
+	if !input.Has() {
+		return false, nil
+	}
+	inputValue := *input.Value()
+
+	formats := []string{
+		"2006-01-02",
+		"2006-01-02 15:04:05",
+		time.RFC3339,
+	}
+	for _, format := range formats {
+		t, err := time.ParseInLocation(format, inputValue, setting.DefaultUILocation)
+		if err == nil {
+			ts := time.Date(t.Year(), t.Month(), t.Day(), 23, 59, 59, 0, setting.DefaultUILocation).Unix()
+			if ts < 86400*2 {
+				ts = 0 // tolerate timezone difference between server and client
+			}
+			*field = timeutil.TimeStamp(ts)
+			return old != *field, nil
+		}
+	}
+	return false, util.NewInvalidArgumentErrorf("invalid date format: %s", inputValue)
 }