Clarify resolveTimeZone null contract and drop unused test import

Addresses Copilot review on PR #2877:
- remove the unused java.util.TimeZone import from ClickHouseColumnTest
  (the test references resolveTimeZone(...)/getTimeZone() only, never the
  TimeZone type)
- document resolveTimeZone's non-null tzName precondition; no behavior
  change (null is unreachable from all parse call sites, which pass a
  non-null replace("'","") result)

Fixes: #2876

Author: polyglotAI-bot

clickhouse-data/src/main/java/com/clickhouse/data/ClickHouseColumn.java

@@ -140,8 +140,8 @@ public enum DefaultValue {
      * converts the offset portion to a {@link ZoneOffset} before creating the
      * {@link TimeZone}, preserving the column's declared fixed offset.
      *
-     * @param tzName timezone name as emitted in ClickHouse column type metadata
-     * @return resolved {@link TimeZone}, never null
+     * @param tzName non-null timezone name as emitted in ClickHouse column type metadata
+     * @return resolved {@link TimeZone}; never null for a non-null {@code tzName}
      */
     static TimeZone resolveTimeZone(String tzName) {
         if (tzName != null && tzName.startsWith("Fixed/UTC")) {

clickhouse-data/src/test/java/com/clickhouse/data/ClickHouseColumnTest.java

@@ -5,7 +5,6 @@
 import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
-import java.util.TimeZone;
 
 import org.testng.Assert;
 import org.testng.annotations.DataProvider;