Repair duration display for bad stopped timestamps (#37121)

Workflow run, job, task, and step durations could show **negative**
values (e.g. `-50s`) when `Stopped` was missing, zero (epoch), or
**before** `Started` (clock skew, races, reruns). The UI used
`calculateDuration` with no validation.

This change:

- Uses each row`s **Updated** timestamp as a **fallback end time** when
`Stopped` is invalid but the status is terminal, so duration
approximates elapsed time instead of `0s` or a negative.
- Keeps **`ActionRun.Duration()`** clamped to **≥ 0** when
`PreviousDuration` plus the current segment would still be negative
(legacy bad data).

Fixes #34582.

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

models/actions/run_test.go

@@ -5,10 +5,12 @@ package actions
 
 import (
 	"testing"
+	"time"
 
 	"code.gitea.io/gitea/models/db"
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/timeutil"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -33,3 +35,13 @@ func TestUpdateRepoRunsNumbers(t *testing.T) {
 	assert.Equal(t, 5, repo.NumActionRuns)
 	assert.Equal(t, 3, repo.NumClosedActionRuns)
 }
+
+func TestActionRun_Duration_NonNegative(t *testing.T) {
+	run := &ActionRun{
+		Started:          timeutil.TimeStamp(100),
+		Stopped:          timeutil.TimeStamp(200),
+		Status:           StatusSuccess,
+		PreviousDuration: -time.Hour,
+	}
+	assert.Equal(t, time.Duration(0), run.Duration())
+}