Clean up dead code, fix misleading comment and test name

- Remove unused VECTOR_OPTION constant from VectorSearchIndex
- Clarify buildKnnQuery() comment: quoted fallback is for forward
  compatibility, all P0 values are already canonicalized as numeric
- Rename testMissingSearchModeOptionThrows to
  testUnknownOptionKeyOnlyThrows to match what it actually tests

Signed-off-by: Eric Wei <mengwei.eric@gmail.com>

Author: mengweieric

opensearch/src/main/java/org/opensearch/sql/opensearch/storage/VectorSearchIndex.java

@@ -23,8 +23,6 @@
  */
 public class VectorSearchIndex extends OpenSearchIndex {
 
-  private static final String VECTOR_OPTION = "vector";
-
   private final String field;
   private final float[] vector;
   private final Map<String, String> options;
@@ -81,7 +79,8 @@ private QueryBuilder buildKnnQuery() {
     for (Map.Entry<String, String> entry : options.entrySet()) {
       optionsJson.append(",");
       String value = entry.getValue();
-      // Numeric values go unquoted, everything else quoted
+      // All P0 option values are canonicalized to numeric strings by validateOptions().
+      // The quoted fallback is retained for forward compatibility with future non-numeric options.
       if (isNumeric(value)) {
         optionsJson.append(String.format("\"%s\":%s", entry.getKey(), value));
       } else {

opensearch/src/test/java/org/opensearch/sql/opensearch/storage/VectorSearchTableFunctionImplementationTest.java

@@ -104,7 +104,7 @@ void testApplyArgumentsWithMinScore() {
   }
 
   @Test
-  void testMissingSearchModeOptionThrows() {
+  void testUnknownOptionKeyOnlyThrows() {
     VectorSearchTableFunctionImplementation impl =
         createImplWithArgs("my-index", "embedding", "[1.0, 2.0]", "not_a_key=100");
     ExpressionEvaluationException ex =