Avoid unnecessary writes during migration repair (#33687)

## Why

Opening a database with current migration history should not require SQLite's
writer slot. The legacy recency repair previously issued an `UPDATE` even when
there was nothing to repair, so it could fail while another connection held the
writer slot.

## What changed

Check `_sqlx_migrations` for the legacy version and checksum before issuing the
repair update, and return early when the current recency migration is already
recorded.

## Testing

Add a WAL-mode regression test that holds the writer slot on a second
connection and verifies that current migration history needs no write access.

GitOrigin-RevId: 76649213912b510df42a836cf8ed7f84a7c70eb5

codex-rs/state/src/migrations.rs

@@ -69,6 +69,27 @@ pub(crate) async fn repair_legacy_recency_migration_version(
         return Ok(());
     }
 
+    let legacy_recency_needs_repair = sqlx::query_scalar::<_, i64>(
+        r#"
+SELECT 1
+FROM _sqlx_migrations
+WHERE version = ?
+  AND checksum = ?
+  AND NOT EXISTS (
+      SELECT 1 FROM _sqlx_migrations WHERE version = ?
+  )
+        "#,
+    )
+    .bind(38_i64)
+    .bind(recency_migration.checksum.as_ref())
+    .bind(recency_migration.version)
+    .fetch_optional(pool)
+    .await?
+    .is_some();
+    if !legacy_recency_needs_repair {
+        return Ok(());
+    }
+
     sqlx::query(
         r#"
 UPDATE _sqlx_migrations