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>

docs/guidelines-backend.md

@@ -62,6 +62,10 @@ Operations that must roll back together should run inside `db.WithTx()` (or
 Functions that participate in a transaction take a `context.Context` as their first
 parameter so the transaction can be propagated.
 
+PostgreSQL, MySQL and MSSQL (via `READ_COMMITTED_SNAPSHOT`) read the last committed
+row version, so reads never wait for writers. Guard read-then-write logic with a
+conditional `UPDATE` or a lock.
+
 ### XORM gotchas
 
 - Never call `x.Update(exemplar)` without an explicit `WHERE` clause — it updates