fix(plugin-search): transaction state errors in parallel reindex operations (#13915)

### What?

Fixed flaky MongoDB transaction state errors ("Attempted illegal state
transition from `[TRANSACTION_ABORTED]` to `[TRANSACTION_COMMITTED]`")
in plugin-search int tests.

### Why?

When reindexing multiple collections in parallel, individual collection
failures were calling `killTransaction()` on a shared
transaction that other parallel operations were still using, causing
MongoDB transaction state conflicts and test flakiness.

### How?

- Moved transaction cleanup to outer catch block only
- Removed individual `killTransaction` calls that created race
conditions
- Allow parallel operations to handle their own errors without aborting
the shared transaction

Author: PatrikKozak

packages/plugin-search/src/utilities/generateReindexHandler.ts

@@ -147,7 +147,7 @@ export const generateReindexHandler =
       }
     }
 
-    await initTransaction(req)
+    const shouldCommit = await initTransaction(req)
 
     try {
       const promises = collections.map(async (collection) => {
@@ -157,14 +157,14 @@ export const generateReindexHandler =
         } catch (err) {
           const message = t('error:unableToReindexCollection', { collection })
           payload.logger.error({ err, msg: message })
-
-          await killTransaction(req)
-          throw new Error(message)
         }
       })
 
       await Promise.all(promises)
     } catch (err: any) {
+      if (shouldCommit) {
+        await killTransaction(req)
+      }
       return Response.json({ message: err.message }, { headers, status: 500 })
     }
 
@@ -174,7 +174,9 @@ export const generateReindexHandler =
       total: aggregateDocs,
     })
 
-    await commitTransaction(req)
+    if (shouldCommit) {
+      await commitTransaction(req)
+    }
 
     return Response.json({ message }, { headers, status: 200 })
   }