fix: use READ_COMMITTED_SNAPSHOT on MSSQL (#39512)

MSSQL's default READ COMMITTED makes reads wait on writers, so the
runner pickup deadlocks with concurrent claims, flaking
`TestCreateTaskForRunnerConcurrentClaim`.

- Enable `READ_COMMITTED_SNAPSHOT` on MSSQL so it reads like PostgreSQL
and MySQL
- Read the pickup cursor before claiming, a lost claim could skip
waiting jobs
- Add tests that fail without consistent READ COMMITTED

Performance: Writes on MSSQL now also store the previous row version in
tempdb, the same versioning cost PostgreSQL and MySQL always pay, and
Azure SQL enables it by default. Reads no longer block on writers, and a
32-runner pickup stress test ran 2.5x faster with it.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

models/actions/task.go

@@ -298,6 +298,12 @@ func CreateTaskForRunner(ctx context.Context, runner *ActionRunner) (*ActionTask
 		if err := e.Where(cond).Asc("updated", "id").Limit(pickTaskBatchSize).Find(&jobs); err != nil {
 			return nil, false, err
 		}
+		// A short page means no waiting jobs remain beyond it.
+		isLastPage := len(jobs) < pickTaskBatchSize
+		if !isLastPage {
+			last := jobs[len(jobs)-1] // read before a lost claim bumps Updated
+			cursorUpdated, cursorID = last.Updated, last.ID
+		}
 
 		for _, v := range jobs {
 			if !runner.CanMatchLabels(v.RunsOn) {
@@ -313,12 +319,9 @@ func CreateTaskForRunner(ctx context.Context, runner *ActionRunner) (*ActionTask
 			// Another runner claimed this job concurrently; try the next one.
 		}
 
-		// A short page means no waiting jobs remain beyond it.
-		if len(jobs) < pickTaskBatchSize {
+		if isLastPage {
 			return nil, false, nil
 		}
-		last := jobs[len(jobs)-1]
-		cursorUpdated, cursorID = last.Updated, last.ID
 	}
 }