Fix DatabaseMetaData.getColumns() returning type name in COLUMN_DEF for columns with no default (#1270)

## Summary

- `COLUMN_DEF` (column 13 of `getColumns()`) was returning the column's
**type name** instead of `null` for columns with no default value,
violating the JDBC spec.
- Root cause: `COLUMN_DEF_COLUMN` was mapped to the `"columnType"`
result-set field, so both the SQL path (default switch case) and the
Thrift path (explicit `case "COLUMN_DEF"`) returned the type name.
- Neither `SHOW COLUMNS` nor the Thrift `TGetColumns` RPC exposes column
default values, so `COLUMN_DEF` must always be `null` for now.
- Fix: added `COLUMN_DEF_COLUMN` to `NULL_COLUMN_COLUMNS` (handles the
Thrift path) and added an explicit `case "COLUMN_DEF": object = null` in
`getRows()` (handles the SQL path).

## Test plan

- added uts
- Tested with legacy driver too : we return null only if it is null, we
don't have a way to determine if default has been set

```
  Driver: DatabricksJDBC v02.07.06.1023
  id:          TYPE_NAME=INT,     COLUMN_DEF=null
  name:        TYPE_NAME=STRING,  COLUMN_DEF=null
  description: TYPE_NAME=VARCHAR, COLUMN_DEF=null
  status:      TYPE_NAME=STRING,  COLUMN_DEF=null  ← has DEFAULT 'active'
  score:       TYPE_NAME=INT,     COLUMN_DEF=null  ← has DEFAULT 0
```

Test : 

```
          String catalog = "samikshya-catalog";
          String schema = "newschema";
          String table = "simba_test_column_def";

 con.createStatement().execute(
                  "CREATE TABLE IF NOT EXISTS `" + catalog + "`.`" + schema +
  "`.`" + table + "`"
                  + " (id INT NOT NULL, name STRING NOT NULL, description
  VARCHAR(255),"
                  + "  status STRING NOT NULL DEFAULT 'active', score INT
  DEFAULT 0)"
                  + " TBLPROPERTIES('delta.feature.allowColumnDefaults' =
  'supported')");

          try (ResultSet rs = con.getMetaData().getColumns(catalog, schema,
  table, null)) {
              while (rs.next()) {
                  System.out.println(rs.getString("COLUMN_NAME")
                          + ": TYPE_NAME=" + rs.getString("TYPE_NAME")
                          + ", COLUMN_DEF=" + rs.getString("COLUMN_DEF"));
              }
          }

          con.createStatement().execute("DROP TABLE IF EXISTS `" + catalog +
  "`.`" + schema + "`.`" + table + "`");
          con.close();
      }
  }
```

Closes #1267

---------

Signed-off-by: samikshya-chand_data <samikshya.chand@databricks.com>

Author: samikshya-db

NEXT_CHANGELOG.md

@@ -18,6 +18,7 @@
 - Fixed `fetchAutoCommitStateFromServer()` to accept both `"1"`/`"0"` and `"true"`/`"false"` responses from `SET AUTOCOMMIT` query, since different server implementations return different formats.
 - Fixed socket leak in SDK HTTP client that prevented CRaC checkpointing. The SDK's connection pool was not shut down on `connection.close()`, leaving TCP sockets open.
 - Fixed Date fields within complex types (ARRAY, STRUCT, MAP) being returned as epoch day integers instead of proper date values.
+- Fixed `DatabaseMetaData.getColumns()` returning the column type name in `COLUMN_DEF` for columns with no default value. `COLUMN_DEF` now correctly returns `null` per the JDBC specification.
 
 ---
 *Note: When making changes, please add your change under the appropriate section

src/main/java/com/databricks/jdbc/dbclient/impl/common/MetadataResultSetBuilder.java

@@ -684,6 +684,10 @@ List<List<Object>> getRows(
               object = null;
             }
             break;
+          case "COLUMN_DEF":
+            // SHOW COLUMNS does not expose column default values; return null per JDBC spec
+            object = null;
+            break;
           default:
             // If column does not match any of the special cases, try to get it from the ResultSet
             try {
@@ -1412,9 +1416,6 @@ List<List<Object>> getThriftRows(List<List<Object>> rows, List<ResultColumn> col
             int ordinalPositionIndex = columns.indexOf(ORDINAL_POSITION_COLUMN);
             object = (int) row.get(ordinalPositionIndex) + 1; // 1-based index
             break;
-          case "COLUMN_DEF":
-            object = row.get(columns.indexOf(COLUMN_TYPE_COLUMN));
-            break;
           default:
             int index = columns.indexOf(column);
             if (index >= row.size()) { // index out of bound (eg: IS_GENERATED_COL in getColumns)

src/test/java/com/databricks/jdbc/dbclient/impl/common/MetadataResultSetBuilderTest.java

@@ -766,4 +766,64 @@ void shouldDenyCatalogAccessWhenCatalogDiffers() throws SQLException {
     assertFalse(metadataResultSetBuilder.shouldAllowCatalogAccess("other", null, session));
     verify(session).getCurrentCatalog();
   }
+
+  @ParameterizedTest
+  @MethodSource("provideColumnDefTypeNames")
+  void testColumnDefIsNullInGetRows(String typeName) throws SQLException {
+    // COLUMN_DEF (index 12 in COLUMN_COLUMNS) must always be null per JDBC spec when no default
+    // is defined; SHOW COLUMNS does not expose column defaults.
+    DatabricksResultSet resultSet = mock(DatabricksResultSet.class);
+    when(resultSet.next()).thenReturn(true).thenReturn(false);
+    for (ResultColumn resultColumn : COLUMN_COLUMNS) {
+      if (resultColumn.getResultSetColumnName().equals("SQLDataType")) {
+        break;
+      }
+      when(resultSet.getObject(resultColumn.getResultSetColumnName())).thenReturn(null);
+    }
+    when(resultSet.getString(COLUMN_TYPE_COLUMN.getResultSetColumnName())).thenReturn(typeName);
+    when(resultSet.getObject(IS_NULLABLE_COLUMN.getResultSetColumnName())).thenReturn("true");
+
+    List<List<Object>> rows =
+        metadataResultSetBuilder.getRows(
+            resultSet, COLUMN_COLUMNS, new DefaultDatabricksResultSetAdapter());
+
+    assertNull(rows.get(0).get(12), "COLUMN_DEF should be null when no default is defined");
+  }
+
+  @ParameterizedTest
+  @MethodSource("provideColumnDefTypeNames")
+  void testColumnDefIsNullInGetThriftRowsWhenNoDefault(String typeName) {
+    // COLUMN_DEF (index 12) must be null when the server returns null for that column.
+    List<Object> row =
+        Arrays.asList(
+            "cat", "schema", "tbl", "col", 4, typeName, 10, null, 0, 10, 1, "", null, null, null,
+            null, 0, "YES", null, null, null, null, "NO", "NO");
+    List<List<Object>> updatedRows =
+        metadataResultSetBuilder.getThriftRows(List.of(row), COLUMN_COLUMNS);
+
+    assertNull(updatedRows.get(0).get(12), "COLUMN_DEF should be null when no default is defined");
+  }
+
+  @Test
+  void testColumnDefIsPreservedInGetThriftRowsWhenDefaultExists() {
+    // COLUMN_DEF (index 12) must return the actual default value from the Thrift response.
+    List<Object> row =
+        Arrays.asList(
+            "cat", "schema", "tbl", "col", 4, "INT", 10, null, 0, 10, 1, "", "'42'", null, null,
+            null, 0, "YES", null, null, null, null, "NO", "NO");
+    List<List<Object>> updatedRows =
+        metadataResultSetBuilder.getThriftRows(List.of(row), COLUMN_COLUMNS);
+
+    assertEquals(
+        "'42'", updatedRows.get(0).get(12), "COLUMN_DEF should return the actual default value");
+  }
+
+  private static Stream<Arguments> provideColumnDefTypeNames() {
+    return Stream.of(
+        Arguments.of("INT"),
+        Arguments.of("STRING"),
+        Arguments.of("VARCHAR(255)"),
+        Arguments.of("DECIMAL(10,2)"),
+        Arguments.of("TIMESTAMP"));
+  }
 }