Add integration test and trim unit-test comments for alias path fix

Add a QueryValidationIT case asserting that SELECT * over an index whose
alias field targets a text multi-field (source.keyword) returns a 400
SemanticCheckException with the descriptive message. An alias pointing at a
truly missing field is rejected by OpenSearch at index-creation time, so it
is not reachable through the SQL plugin and is covered by the unit test only.

Shorten the unit-test comments and drop inline issue references.

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

Author: RyanL1997

integ-test/src/test/java/org/opensearch/sql/sql/QueryValidationIT.java

@@ -103,6 +103,33 @@ private static void whenExecuteMalformedPayload() throws IOException {
     client().performRequest(request);
   }
 
+  // An alias field whose path targets a text multi-field (e.g. "source.keyword") must fail with a
+  // descriptive client error rather than an opaque NullPointerException.
+  @Test
+  public void testAliasToKeywordMultiFieldFailsWithBadRequest() throws IOException {
+    String index = "test_alias_unresolved_keyword";
+    createIndexWithMapping(
+        index,
+        "{ \"properties\": {"
+            + "  \"source\": { \"type\": \"text\", \"fields\": { \"keyword\": { \"type\":"
+            + " \"keyword\" } } },"
+            + "  \"source_alias\": { \"type\": \"alias\", \"path\": \"source.keyword\" } } }");
+
+    expectResponseException()
+        .hasStatusCode(BAD_REQUEST)
+        .hasErrorType("SemanticCheckException")
+        .containsMessage(
+            "Alias field [source_alias] refers to unresolved path [source.keyword]")
+        .whenExecute(String.format(Locale.ROOT, "SELECT * FROM %s", index));
+  }
+
+  private static void createIndexWithMapping(String indexName, String mapping)
+      throws IOException {
+    Request request = new Request("PUT", "/" + indexName);
+    request.setJsonEntity(String.format(Locale.ROOT, "{ \"mappings\": %s }", mapping));
+    client().performRequest(request);
+  }
+
   public ResponseExceptionAssertion expectResponseException() {
     return new ResponseExceptionAssertion(exceptionRule);
   }

opensearch/src/test/java/org/opensearch/sql/opensearch/data/type/OpenSearchDataTypeTest.java

@@ -486,10 +486,7 @@ public void test_AliasType() {
 
   @Test
   public void traverseAndFlatten_alias_to_unresolvable_path_throws_descriptive_error() {
-    // An alias whose path targets a text multi-field (e.g. "source.keyword"). Multi-fields are
-    // stored under OpenSearchTextType.fields, not properties, so they are never added to the
-    // flattened mapping and the alias target resolves to null. Previously this surfaced as an
-    // opaque NullPointerException (issue #5535).
+    // Alias path targets a text multi-field, which is not in the flattened mapping.
     Map<String, OpenSearchDataType> keywordAliasTree =
         Map.of(
             "source", textKeywordType,
@@ -505,7 +502,7 @@ public void traverseAndFlatten_alias_to_unresolvable_path_throws_descriptive_err
             + " \"source.keyword.keyword\") or a removed/renamed field is not a valid alias target.",
         keywordException.getMessage());
 
-    // An alias whose path targets a field that does not exist (e.g. renamed/removed).
+    // Alias path targets a field that does not exist.
     Map<String, OpenSearchDataType> missingFieldTree =
         Map.of(
             "col1", textType,