[SPARK-56469][SQL][TESTS] BufferedRowsReader lifecycle hardening

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

Extracted from #55116.

The test-framework reader in `InMemoryBaseTable` now tracks a closed flag and throws `IllegalStateException` for reads, double-closes, or metric fetches on a closed reader. This ensures future tests catch reader lifecycle bugs that were previously hidden by the noop `close()`.

### Why are the changes needed?

Improve test framework.

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

No.

### How was this patch tested?

Existing UTs.

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

No.

Closes #55336 from peter-toth/SPARK-56469-bufferedrowsreader-lifecycle-hardening.

Authored-by: Peter Toth <peter.toth@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: peter-toth

sql/catalyst/src/test/scala/org/apache/spark/sql/connector/catalog/InMemoryBaseTable.scala

@@ -877,15 +877,21 @@ private class BufferedRowsReader(
 
   private var index: Int = -1
   private var rowsRead: Long = 0
+  private var closed: Boolean = false
+
+  private def checkNotClosed(op: String): Unit =
+    if (closed) throw new IllegalStateException(s"$op called on a closed BufferedRowsReader")
 
   override def next(): Boolean = {
+    checkNotClosed("next()")
     index += 1
     val hasNext = index < partition.rows.length
     if (hasNext) rowsRead += 1
     hasNext
   }
 
   override def get(): InternalRow = {
+    checkNotClosed("get()")
     val originalRow = partition.rows(index)
     val values = new Array[Any](nonMetadataColumns.length)
     nonMetadataColumns.zipWithIndex.foreach { case (col, idx) =>
@@ -895,7 +901,13 @@ private class BufferedRowsReader(
     addMetadata(new GenericInternalRow(values))
   }
 
-  override def close(): Unit = {}
+  // Intentionally strict: double-close throws rather than being idempotent (as Closeable permits).
+  // This is test code whose purpose is to catch reader lifecycle bugs early; a silent no-op on
+  // double-close would mask the very errors we want to detect.
+  override def close(): Unit = {
+    checkNotClosed("close()")
+    closed = true
+  }
 
   private def extractFieldValue(
       field: StructField,
@@ -1041,6 +1053,7 @@ private class BufferedRowsReader(
   }
 
   override def currentMetricsValues(): Array[CustomTaskMetric] = {
+    checkNotClosed("currentMetricsValues()")
     val metric = new CustomTaskMetric {
       override def name(): String = "rows_read"
       override def value(): Long = rowsRead