Migrate core temporal models and related entities to java.time.Instant (#3001)

This comprehensive refactor continues the migration from Joda-Time to java.time (Instant), focusing on core timestamp models, transition properties, and their integration across the codebase.

Key changes:
- Migrated CreateAutoTimestamp and UpdateAutoTimestamp to use Instant internally, providing Joda-Time bridge methods for backward compatibility.
- Updated TimedTransitionProperty to handle Instant-based transition maps and updated corresponding Hibernate UserTypes (TimedTransitionBaseUserType).
- Migrated GracePeriod, BillingBase, BillingEvent, PollMessage, and PendingActionNotificationResponse fields (e.g., expirationTime, eventTime) to Instant.
- Migrated additional core entities (DomainBase, Registrar, HostBase, LaunchNotice, BsaLabel, DomainTransactionRecord) to use Instant for registrationExpirationTime, lastTransferTime, creationTime, etc.
- Updated Tld and FeatureFlag models to use Instant for claimsPeriodEnd, bsaEnrollStartTime, and status transitions.
- Enhanced CLI tools and parameters (TransitionListParameter, InstantParameter, RequestParameters) to support Instant-based input and output.
- Updated EntityYamlUtils with custom Instant serializers/deserializers to maintain format consistency (e.g., .SSSZ precision) required for YAML-based tests.
- Implemented UtcInstantAdapter to ensure JAXB XML serialization maintains millisecond accuracy, matching legacy Joda-Time behavior.
- Resolved Hibernate 6 type mismatches in JPQL and Native queries by ensuring consistent use of Instant for comparisons.
- Updated GEMINI.md with project-specific engineering standards, including the 'one commit per PR' mandate, full-build validation requirement, and commit message style rules.
- Cleaned up unnecessary @JsonIgnore and @JsonProperty annotations that were previously added to methods with parameters or redundant fields.
- Refactored DateTimeUtils to use strongly-typed overloads and standardized naming (earliestOf, latestOf) while avoiding type erasure clashes.
- Cleaned up fully qualified calls to toDateTime and toInstant by adding static imports across core model and flow files.
- Refactored test suites to use clock.now() (Instant) instead of nowUtc() (DateTime) and removed custom Truth subjects in favor of standard assertions.

common/src/testing/java/google/registry/testing/FakeClock.java

@@ -49,6 +49,11 @@ public final class FakeClock implements Clock {
     setTo(startTime);
   }
 
+  /** Creates a FakeClock initialized to a specific time. */
+  public FakeClock(Instant startTime) {
+    setTo(startTime);
+  }
+
   /** Returns the current time. */
   @Override
   public DateTime nowUtc() {
@@ -89,6 +94,11 @@ public final class FakeClock implements Clock {
     currentTimeMillis.set(time.getMillis());
   }
 
+  /** Sets the time to the specified instant. */
+  public void setTo(Instant time) {
+    currentTimeMillis.set(time.toEpochMilli());
+  }
+
   /** Invokes {@link #setAutoIncrementStep} with one millisecond-step. */
   public FakeClock setAutoIncrementByOneMilli() {
     return setAutoIncrementStep(Duration.millis(1));