Fix leap second overflow in datetime parsing (#1015)

Normalize leap seconds (second=60) to second=59 before passing to
time.Date() to prevent year overflow. When Go's time.Date() receives
second=60, it normalizes the time by adding 1 minute, which can cause
dates like 9999-12-31 23:59:60 to overflow to year 10000 - outside
the valid TOML date range (0000-9999).

This fix ensures that timestamps with leap seconds can be successfully
round-tripped (parsed and re-serialized) without causing parsing errors.

Fixes OSS-Fuzz issue 472183443

localtime.go

@@ -94,7 +94,13 @@ type LocalDateTime struct {
 
 // AsTime converts d into a specific time instance in zone.
 func (d LocalDateTime) AsTime(zone *time.Location) time.Time {
-	return time.Date(d.Year, time.Month(d.Month), d.Day, d.Hour, d.Minute, d.Second, d.Nanosecond, zone)
+	// Normalize leap seconds (second=60) to second=59 to prevent overflow
+	// when Go's time.Date normalizes the time.
+	second := d.Second
+	if second == 60 {
+		second = 59
+	}
+	return time.Date(d.Year, time.Month(d.Month), d.Day, d.Hour, d.Minute, second, d.Nanosecond, zone)
 }
 
 // String returns RFC 3339 representation of d.