[SPARK-49872][FOLLOWUP] Remove Jackson JSON string length limit in KVStoreScalaSerializer

### What changes were proposed in this pull request?

Along the same lines as #52049, apply the same fix to `KVStoreScalaSerializer` as well.

### Why are the changes needed?

The previous fix in #52049 only removed the Jackson JSON string length limit in `JsonProtocol`, which covered Spark event <=> JSON SerDe.

However, the exact same problem exists in `KVStoreScalaSerializer` as well, which is triggered when SHS is configured to use LevelDB/RocksDB KVStores.

Example call chain that triggers the issue:
```
  ExecutionPage.render()
    → sqlStore.planGraph(executionId, version)
      → store.read(classOf[SparkPlanGraphWrapper], ...)
        → KVStoreSerializer.deserialize()
          → mapper.readValue()  // ← FAILS HERE with 20MB limit
```

### Does this PR introduce _any_ user-facing change?

Yes, users won't hit the Jackson JSON string length limit any more when using SHS.

### How was this patch tested?

Manually tested with a SQL workload that had an intentionally long string for the plan graph.
The fix mechanism itself is tested by previous PRs #49163 and #52049, this is just applying the same fix to SHS as well.

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #53711 from rednaxelafx/fix-jackson-strlenlimit-kvstore.

Authored-by: Kris Mok <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: rednaxelafx

core/src/main/scala/org/apache/spark/status/KVUtils.scala

@@ -25,6 +25,7 @@ import scala.jdk.CollectionConverters._
 import scala.reflect.{classTag, ClassTag}
 
 import com.fasterxml.jackson.annotation.JsonInclude
+import com.fasterxml.jackson.core.StreamReadConstraints
 import com.fasterxml.jackson.module.scala.DefaultScalaModule
 import org.fusesource.leveldbjni.internal.NativeDB
 import org.rocksdb.RocksDBException
@@ -76,6 +77,10 @@ private[spark] object KVUtils extends Logging {
     mapper.registerModule(DefaultScalaModule)
     mapper.setDefaultPropertyInclusion(JsonInclude.Include.NON_ABSENT)
 
+    // SPARK-49872: Remove jackson JSON string length limitation.
+    mapper.getFactory.setStreamReadConstraints(
+      StreamReadConstraints.builder().maxStringLength(Int.MaxValue).build()
+    )
   }
 
   /**