Add integration test for rex in subsearch + simplify unit-test javadoc

Adds CalcitePPLAppendcolIT.testAppendColWithRexInSubsearch to cover the
end-to-end execution path against a real cluster. Trims the existing
unit-test javadoc to a single line describing what is being verified,
not the historical bug shape.

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

integ-test/src/test/java/org/opensearch/sql/calcite/remote/CalcitePPLAppendcolIT.java

@@ -81,4 +81,18 @@ public void testAppendColOverride() throws IOException {
         rows("F", "DE", 101, null),
         rows("F", "FL", 310, null));
   }
+
+  /** Verifies that rex can be used as the first command of an appendcol subsearch. */
+  @Test
+  public void testAppendColWithRexInSubsearch() throws IOException {
+    JSONObject actual =
+        executeQuery(
+            String.format(
+                "source=%s | stats count() as cnt by gender"
+                    + " | appendcol [ rex field=email '^(?<user>[^@]+)@.*' | fields user ]"
+                    + " | head 2",
+                TEST_INDEX_ACCOUNT));
+    verifySchema(
+        actual, schema("gender", "string"), schema("cnt", "bigint"), schema("user", "string"));
+  }
 }

ppl/src/test/java/org/opensearch/sql/ppl/calcite/CalcitePPLRexTest.java

@@ -307,24 +307,13 @@ public void testRexWithMaxMatchAndOffsetField() {
     verifyPPLToSparkSQL(root, expectedSparkSql);
   }
 
-  /**
-   * Regression for the rex-inside-appendcol NPE. The subsearch starts with rex (no upstream
-   * source), so the parsed Rex node has a null {@code child} until {@link
-   * org.opensearch.sql.calcite.utils.PlanUtils#transformPlanToAttachChild} walks the subsearch tree
-   * to attach the main relation. That walker calls {@code getChild()} on every node it crosses.
-   * {@code Rex.getChild()} previously returned {@code ImmutableList.of(child)} which throws NPE
-   * when {@code child} is null; sibling AST nodes (Project / Filter / Eval / Aggregation) all guard
-   * with {@code child == null ? ImmutableList.of() : ...}. This test ensures the rex-in-appendcol
-   * pipeline plans without NPE.
-   */
+  /** Verifies that rex plans correctly when it appears as the first command of a subsearch. */
   @Test
-  public void testRexInsideAppendCol() {
+  public void testRexInsideSubsearch() {
     String ppl =
         "source=EMP | stats count() as base_c by JOB"
             + " | appendcol [ rex field=ENAME '^(?<first>[A-Z])' | fields ENAME, first ]";
     RelNode root = getRelNode(ppl);
-    // Just verify planning succeeds (does not throw). Plan-shape assertions for AppendCol's join
-    // shape live in CalcitePPLAppendColumnsTest; this regression only covers the NPE.
     org.junit.Assert.assertNotNull(root);
   }
 }