fix: Emit fixed microsecond precision for JSR-310 temporals

Java's OffsetDateTime/Instant/ZonedDateTime/LocalDateTime/LocalTime
toString() omits seconds and fractional digits when zero, producing
inconsistent serialised forms (e.g. "20:00Z" vs "20:00:00.123456Z").
Encode through a DateTimeFormatter that always writes six-digit
(microsecond) fractional precision. Decoding remains permissive via
each type's ISO parser.

Author: thced

src/main/java/com/retailsvc/http/internal/gson/GsonJsonMapper.java

@@ -20,7 +20,11 @@
 import java.time.LocalDateTime;
 import java.time.LocalTime;
 import java.time.OffsetDateTime;
+import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
+import java.time.format.DateTimeFormatter;
+import java.time.format.DateTimeFormatterBuilder;
+import java.time.temporal.ChronoField;
 import java.util.ArrayList;
 import java.util.LinkedHashMap;
 import java.util.List;
@@ -40,8 +44,9 @@
  * {@code Instant}, etc.).
  *
  * <p>JSR-310 types ({@code Instant}, {@code OffsetDateTime}, {@code ZonedDateTime}, {@code
- * LocalDateTime}, {@code LocalDate}, {@code LocalTime}) are round-tripped as their ISO-8601 string
- * form.
+ * LocalDateTime}, {@code LocalDate}, {@code LocalTime}) are round-tripped as ISO-8601 strings.
+ * Types with a sub-second component are serialised with fixed six-digit (microsecond) fractional
+ * precision; parsing accepts any ISO-8601 fractional precision (or none).
  */
 public final class GsonJsonMapper implements TypedTypeMapper {
 
@@ -60,17 +65,47 @@ public Gson gson() {
     return gson;
   }
 
+  private static final DateTimeFormatter LOCAL_TIME_MICROS =
+      new DateTimeFormatterBuilder()
+          .appendPattern("HH:mm:ss")
+          .appendFraction(ChronoField.NANO_OF_SECOND, 6, 6, true)
+          .toFormatter();
+
+  private static final DateTimeFormatter LOCAL_DATE_TIME_MICROS =
+      new DateTimeFormatterBuilder()
+          .append(DateTimeFormatter.ISO_LOCAL_DATE)
+          .appendLiteral('T')
+          .append(LOCAL_TIME_MICROS)
+          .toFormatter();
+
+  private static final DateTimeFormatter OFFSET_DATE_TIME_MICROS =
+      new DateTimeFormatterBuilder().append(LOCAL_DATE_TIME_MICROS).appendOffsetId().toFormatter();
+
+  private static final DateTimeFormatter ZONED_DATE_TIME_MICROS =
+      new DateTimeFormatterBuilder()
+          .append(OFFSET_DATE_TIME_MICROS)
+          .optionalStart()
+          .appendLiteral('[')
+          .parseCaseSensitive()
+          .appendZoneRegionId()
+          .appendLiteral(']')
+          .optionalEnd()
+          .toFormatter();
+
+  private static final DateTimeFormatter INSTANT_MICROS =
+      OFFSET_DATE_TIME_MICROS.withZone(ZoneOffset.UTC);
+
   private static Gson defaultGson() {
     return new GsonBuilder()
-        .registerTypeAdapter(Instant.class, iso(Instant::toString, Instant::parse))
+        .registerTypeAdapter(Instant.class, iso(INSTANT_MICROS::format, Instant::parse))
         .registerTypeAdapter(
-            OffsetDateTime.class, iso(OffsetDateTime::toString, OffsetDateTime::parse))
+            OffsetDateTime.class, iso(OFFSET_DATE_TIME_MICROS::format, OffsetDateTime::parse))
         .registerTypeAdapter(
-            ZonedDateTime.class, iso(ZonedDateTime::toString, ZonedDateTime::parse))
+            ZonedDateTime.class, iso(ZONED_DATE_TIME_MICROS::format, ZonedDateTime::parse))
         .registerTypeAdapter(
-            LocalDateTime.class, iso(LocalDateTime::toString, LocalDateTime::parse))
+            LocalDateTime.class, iso(LOCAL_DATE_TIME_MICROS::format, LocalDateTime::parse))
         .registerTypeAdapter(LocalDate.class, iso(LocalDate::toString, LocalDate::parse))
-        .registerTypeAdapter(LocalTime.class, iso(LocalTime::toString, LocalTime::parse))
+        .registerTypeAdapter(LocalTime.class, iso(LOCAL_TIME_MICROS::format, LocalTime::parse))
         .create();
   }
 

src/test/java/com/retailsvc/http/internal/gson/GsonJsonMapperTest.java

@@ -61,21 +61,21 @@ void writesMapAndList() {
   void writesInstantAsIso8601() {
     Instant t = Instant.parse("2026-05-13T10:00:00Z");
     assertThat(new String(mapper.writeTo(Map.of("ts", t)), StandardCharsets.UTF_8))
-        .isEqualTo("{\"ts\":\"2026-05-13T10:00:00Z\"}");
+        .isEqualTo("{\"ts\":\"2026-05-13T10:00:00.000000Z\"}");
   }
 
   @Test
   void writesOffsetDateTimeAsIso8601() {
     OffsetDateTime t = OffsetDateTime.of(2026, 5, 13, 10, 0, 0, 0, ZoneOffset.UTC);
     assertThat(new String(mapper.writeTo(Map.of("ts", t)), StandardCharsets.UTF_8))
-        .isEqualTo("{\"ts\":\"2026-05-13T10:00Z\"}");
+        .isEqualTo("{\"ts\":\"2026-05-13T10:00:00.000000Z\"}");
   }
 
   @Test
   void writesZonedDateTimeAsIso8601() {
     ZonedDateTime t = ZonedDateTime.of(2026, 5, 13, 10, 0, 0, 0, ZoneOffset.UTC);
     assertThat(new String(mapper.writeTo(Map.of("ts", t)), StandardCharsets.UTF_8))
-        .contains("2026-05-13T10:00Z");
+        .contains("2026-05-13T10:00:00.000000Z");
   }
 
   @Test
@@ -84,7 +84,7 @@ void writesLocalDateTimeAsIso8601() {
             new String(
                 mapper.writeTo(Map.of("ts", LocalDateTime.of(2026, 5, 13, 10, 0))),
                 StandardCharsets.UTF_8))
-        .isEqualTo("{\"ts\":\"2026-05-13T10:00\"}");
+        .isEqualTo("{\"ts\":\"2026-05-13T10:00:00.000000\"}");
   }
 
   @Test
@@ -98,7 +98,7 @@ void writesLocalDateAsIso8601() {
   @Test
   void writesLocalTimeAsIso8601() {
     assertThat(new String(mapper.writeTo(Map.of("t", LocalTime.of(10, 0))), StandardCharsets.UTF_8))
-        .isEqualTo("{\"t\":\"10:00\"}");
+        .isEqualTo("{\"t\":\"10:00:00.000000\"}");
   }
 
   @Test