Add methods to return subtypes of HistoryEntry when querying (#1172)

This is useful when we expect a specific subtype in the return value so
that we can set the parent resource (e. g. DomainContent for
DomainHistory) on it, or when a specific subtype is needed from the call
site.

This PR also fixes some use of generic return values. It is always better to
return <HistoryEntry> than a wildcard <? extends HistoryEntry>, because for
immutable collections, <? extends HistoryEntry> is no different than
<HistoryEntry> as return value -- you can only get a HistoryEntry from it.
The wildcard return value means that even if you are indeed getting a
<DomainHistory> from the query, the call site has no compile time knowledge of
it and can only assume it is a <HistoryEntry>.

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core/src/test/java/google/registry/testing/DatabaseHelper.java

@@ -1101,10 +1101,19 @@ public class DatabaseHelper {
   }
 
   /** Returns all of the history entries that are parented off the given EppResource. */
-  public static List<? extends HistoryEntry> getHistoryEntries(EppResource resource) {
+  public static List<HistoryEntry> getHistoryEntries(EppResource resource) {
     return HistoryEntryDao.loadHistoryObjectsForResource(resource.createVKey());
   }
 
+  /**
+   * Returns all of the history entries that are parented off the given EppResource, casted to the
+   * corresponding subclass.
+   */
+  public static <T extends HistoryEntry> List<T> getHistoryEntries(
+      EppResource resource, Class<T> subclazz) {
+    return HistoryEntryDao.loadHistoryObjectsForResource(resource.createVKey(), subclazz);
+  }
+
   /**
    * Returns all of the history entries that are parented off the given EppResource with the given
    * type.
@@ -1112,7 +1121,18 @@ public class DatabaseHelper {
   public static ImmutableList<HistoryEntry> getHistoryEntriesOfType(
       EppResource resource, final HistoryEntry.Type type) {
     return getHistoryEntries(resource).stream()
-        .filter(entry -> entry.getType() == type)
+        .filter(entry -> entry.getType().equals(type))
+        .collect(toImmutableList());
+  }
+
+  /**
+   * Returns all of the history entries that are parented off the given EppResource with the given
+   * type and casted to the corresponding subclass.
+   */
+  public static <T extends HistoryEntry> ImmutableList<T> getHistoryEntriesOfType(
+      EppResource resource, final HistoryEntry.Type type, Class<T> subclazz) {
+    return getHistoryEntries(resource, subclazz).stream()
+        .filter(entry -> entry.getType().equals(type))
         .collect(toImmutableList());
   }
 
@@ -1122,9 +1142,16 @@ public class DatabaseHelper {
    */
   public static HistoryEntry getOnlyHistoryEntryOfType(
       EppResource resource, final HistoryEntry.Type type) {
-    List<HistoryEntry> historyEntries = getHistoryEntriesOfType(resource, type);
-    assertThat(historyEntries).hasSize(1);
-    return historyEntries.get(0);
+    return Iterables.getOnlyElement(getHistoryEntriesOfType(resource, type));
+  }
+
+  /**
+   * Returns the only history entry of the given type, casted to the corresponding subtype, and
+   * throws an AssertionError if there are zero or more than one.
+   */
+  public static <T extends HistoryEntry> T getOnlyHistoryEntryOfType(
+      EppResource resource, final HistoryEntry.Type type, Class<T> subclazz) {
+    return Iterables.getOnlyElement(getHistoryEntriesOfType(resource, type, subclazz));
   }
 
   private static HistoryEntry.Type getHistoryEntryType(EppResource resource) {