Add nanosecond-precision round-trip test for INT96 stats

Existing tests only used .000 millisecond timestamps, so the comparator's
8-byte unsigned-long nanos-of-day path was never exercised below minute
granularity. New test covers same-day timestamps at 1ns, 1us, 1ms,
mid-range, and max-of-day (23:59:59.999999999).

Author: rahulketch

parquet-hadoop/src/test/java/org/apache/parquet/statistics/TestInt96TimestampStatisticsRoundTrip.java

@@ -145,4 +145,16 @@ public void testIncreasingDayDecreasingTime() throws IOException {
     String[] timestamps = {"2020-01-01T12:00:00.000", "2020-02-01T11:00:00.000", "2020-03-01T10:00:00.000"};
     runTimestampTest(timestamps);
   }
+
+  @Test
+  public void testSameDayNanosecondPrecision() throws IOException {
+    String[] timestamps = {
+      "2020-01-01T00:00:00.000000001",
+      "2020-01-01T00:00:00.000001000",
+      "2020-01-01T00:00:00.001000000",
+      "2020-01-01T12:34:56.123456789",
+      "2020-01-01T23:59:59.999999999"
+    };
+    runTimestampTest(timestamps);
+  }
 }