Fix charset mismatch by overriding getDefaultCharset() with UTF-8

The previous fix patched createSqlType() for the no-arg and boolean
variants, but Calcite has many code paths for char type creation:
  - createSqlType(SqlTypeName, int precision)
  - RexBuilder.makeLiteral(String) → getDefaultCharset()
  - RelBuilder.literal(String) → getDefaultCharset()

All of these bypassed the per-method overrides, causing residual
'VARCHAR CHARACTER SET UTF-8 is not comparable to CHAR(1)' errors
in RangeFormatter and other callers (e.g. bin command).

Fix: override getDefaultCharset() in OpenSearchTypeFactory to return
UTF-8. This is the single source of truth Calcite uses across all
char type creation paths, making every VARCHAR/CHAR consistently
UTF-8 without needing per-call patches.

The per-method createSqlType overrides are removed as redundant.

Signed-off-by: Radhakrishnan Pachyappan <gingeekrishna@gmail.com>

Author: gingeekrishna

core/src/main/java/org/opensearch/sql/calcite/utils/OpenSearchTypeFactory.java

@@ -96,6 +96,11 @@ public RelDataType createTypeWithNullability(RelDataType type, boolean nullable)
     return super.createTypeWithNullability(type, nullable);
   }
 
+  @Override
+  public Charset getDefaultCharset() {
+    return StandardCharsets.UTF_8;
+  }
+
   @Override
   public RelDataType createTypeWithCharsetAndCollation(
       RelDataType type, Charset charset, SqlCollation collation) {
@@ -105,21 +110,8 @@ public RelDataType createTypeWithCharsetAndCollation(
     return super.createTypeWithCharsetAndCollation(type, charset, collation);
   }
 
-  @Override
-  public RelDataType createSqlType(SqlTypeName typeName) {
-    RelDataType type = super.createSqlType(typeName);
-    if (typeName == SqlTypeName.VARCHAR || typeName == SqlTypeName.CHAR) {
-      return createTypeWithCharsetAndCollation(type, StandardCharsets.UTF_8, SqlCollation.IMPLICIT);
-    }
-    return type;
-  }
-
   public RelDataType createSqlType(SqlTypeName typeName, boolean nullable) {
-    RelDataType type = createTypeWithNullability(super.createSqlType(typeName), nullable);
-    if (typeName == SqlTypeName.VARCHAR || typeName == SqlTypeName.CHAR) {
-      return createTypeWithCharsetAndCollation(type, StandardCharsets.UTF_8, SqlCollation.IMPLICIT);
-    }
-    return type;
+    return createTypeWithNullability(super.createSqlType(typeName), nullable);
   }
 
   public RelDataType createStructType(