Return 4xx instead of 500 for unsupported window functions

Window functions outside WINDOW_FUNC_MAPPING (e.g. RANK) used to escape
the AE route as HTTP 500 because the throw site emitted a raw
UnsupportedOperationException, which UnifiedQueryPlanner rethrows
unchanged. Switching to CalciteUnsupportedException lets the existing
4xx wrapper added in #5569 normalize it to SemanticCheckException.

Repro (Mustang bug-bash query, AE route):
  SELECT RegionID, COUNT(*) AS cnt,
         RANK() OVER (ORDER BY COUNT(*) DESC) AS rnk
  FROM clickbench GROUP BY RegionID LIMIT 5

Signed-off-by: Michael Oviedo <mikeovi@amazon.com>

Author: OVI3D0

api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerTest.java

@@ -159,6 +159,18 @@ public void unsupportedFeatureIsRethrownAsSemanticCheckException() {
         .assertErrorMessageContains("unsupported in Calcite");
   }
 
+  @Test
+  public void unsupportedWindowFunctionIsRethrownAsSemanticCheckException() {
+    // Window functions outside WINDOW_FUNC_MAPPING (e.g. RANK, used by Mustang bug-bash query
+    // `RANK() OVER (ORDER BY COUNT(*) DESC)`) reach CalciteRexNodeVisitor#visitWindowFunction's
+    // orElseThrow. The throw site emits CalciteUnsupportedException so this path normalizes to a
+    // 4xx SemanticCheckException rather than escaping as a 500.
+    givenInvalidQuery("source = catalog.employees | eventstats rank()")
+        .assertErrorType(SemanticCheckException.class)
+        .assertCauseType(CalciteUnsupportedException.class)
+        .assertErrorMessageContains("Unexpected window function: rank");
+  }
+
   @Test
   public void assertionErrorIsWrappedAsSemanticCheckException() {
     // Remove when the underlying Calcite assertion is fixed.

core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java

@@ -705,7 +705,7 @@ public RexNode visitWindowFunction(WindowFunction node, CalcitePlanContext conte
                       node.getWindowFrame());
             })
         .orElseThrow(
-            () -> new UnsupportedOperationException("Unexpected window function: " + funcName));
+            () -> new CalciteUnsupportedException("Unexpected window function: " + funcName));
   }
 
   private List<RexNode> translateOrderKeys(