Improve ISODuration string formatting (#4123)

- Use `strconv.FormatFloat` for consistent output
- Add tests for rounding and formatting edge cases

util/timeutil/isoduration.go

@@ -112,7 +112,7 @@ func (dur ISODuration) String() string {
 		sec := dur.SecondPart
 		// round to microseconds
 		sec = math.Round(sec*1e6) / 1e6
-		fmt.Fprintf(&b, "%gS", sec)
+		fmt.Fprintf(&b, "%sS", strconv.FormatFloat(sec, 'f', -1, 64))
 	}
 
 	return b.String()

util/timeutil/isoduration_test.go

@@ -23,6 +23,8 @@ func TestISODuration_String(t *testing.T) {
 	check("P1D", ISODuration{DayPart: 1})
 	check("PT1H", ISODuration{HourPart: 1})
 	check("P1YT0.1S", ISODuration{YearPart: 1, SecondPart: 0.1})
+	check("P1YT0.123457S", ISODuration{YearPart: 1, SecondPart: 0.1234567})           // Note: rounded to 6 decimal places, so the 6th digit is 7 instead of 6.
+	check("P1YT0.000076S", ISODuration{YearPart: 1, SecondPart: 7.5809468881749e-05}) // Ensure we don't print an exponent.
 
 	check("P1Y2M3W4DT5H6M7S", ISODuration{
 		YearPart:  1,
@@ -39,7 +41,6 @@ func TestISODuration_String(t *testing.T) {
 }
 
 func TestParseISODuration(t *testing.T) {
-
 	check := func(desc string, iso string, exp ISODuration) {
 		t.Helper()