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/main/java/google/registry/util/SystemClock.java

@@ -18,6 +18,7 @@ import static org.joda.time.DateTimeZone.UTC;
 
 import jakarta.inject.Inject;
 import java.time.Instant;
+import java.time.temporal.ChronoUnit;
 import javax.annotation.concurrent.ThreadSafe;
 import org.joda.time.DateTime;
 
@@ -42,6 +43,6 @@ public class SystemClock implements Clock {
     // (which uses millisecond precision via DateTimeConverter). This prevents subtle comparison
     // bugs where a high-precision Instant would be considered "after" a truncated database
     // timestamp.
-    return Instant.now().truncatedTo(java.time.temporal.ChronoUnit.MILLIS);
+    return Instant.now().truncatedTo(ChronoUnit.MILLIS);
   }
 }