Make relationshipLink a terminal navigator

Previously relationshipLink set a field on the navigator and returned itself, so it
could coexist with a navigated relationship and be mutated further, allowing
nonsensical paths. It now returns a dedicated terminal type
(ElideNavigatorOnRelationshipLink) that only exposes build(), builds the path
eagerly, and rejects includes on the parent (mirroring navigateRelationship).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Brutus5000

api/src/main/java/com/faforever/commons/api/elide/ElideNavigator.java

@@ -27,7 +27,6 @@ public class ElideNavigator<T extends ElideEntity> implements ElideNavigatorSele
   private final Optional<ElideNavigator<?>> parentNavigator;
   private Optional<String> id = Optional.empty();
   private Optional<String> relationship = Optional.empty();
-  private Optional<String> relationshipLink = Optional.empty();
   private Optional<Condition<?>> filterCondition = Optional.empty();
   private Optional<Integer> pageSize = Optional.empty();
   private Optional<Integer> pageNumber = Optional.empty();
@@ -116,10 +115,13 @@ public <R extends ElideEntity> ElideNavigatorSelector<R> navigateRelationship(@N
   }
 
   @Override
-  public ElideNavigatorOnId<T> relationshipLink(@NotNull String name) {
+  public ElideNavigatorOnRelationshipLink relationshipLink(@NotNull String name) {
+    if (!includes.isEmpty()) {
+      throw new IllegalStateException("Cannot navigate to a relationship link with includes on parent");
+    }
     log.trace("relationship link added: {}", name);
-    this.relationshipLink = Optional.of(name);
-    return this;
+    String route = build() + "/relationships/" + name;
+    return () -> route;
   }
 
   /**
@@ -197,7 +199,6 @@ public String build() {
     String route = parentNavigator.map(ElideNavigator::build).orElse("/data/" + dtoPath) +
                    id.map(i -> "/" + i).orElse("") +
                    relationship.map(r -> "/" + r).orElse("") +
-                   relationshipLink.map(r -> "/relationships/" + r).orElse("") +
                    queryArgs;
     log.trace("Route built: {}", route);
     return route;

api/src/main/java/com/faforever/commons/api/elide/ElideNavigatorOnId.java

@@ -11,7 +11,10 @@ public interface ElideNavigatorOnId<T extends ElideEntity> extends ElideEndpoint
    * ({@code /data/{type}/{id}/relationships/{name}}), used to read or modify the relationship linkage itself
    * (e.g. PATCH/POST/DELETE to add, replace or remove members). This differs from
    * {@link #navigateRelationship(Class, String)}, which addresses the related resource(s).
+   *
+   * <p>This is a terminal operation: the returned navigator only allows {@link
+   * ElideNavigatorOnRelationshipLink#build()}. Includes are not allowed on the parent.
    */
-  ElideNavigatorOnId<T> relationshipLink(String name);
+  ElideNavigatorOnRelationshipLink relationshipLink(String name);
 
 }

api/src/main/java/com/faforever/commons/api/elide/ElideNavigatorOnRelationshipLink.java

@@ -0,0 +1,11 @@
+package com.faforever.commons.api.elide;
+
+/**
+ * Terminal navigator pointing at a JSON:API relationship endpoint
+ * ({@code /data/{type}/{id}/relationships/{name}}). It only exposes {@link #build()} because a relationship
+ * link addresses the linkage itself (read or modify via PATCH/POST/DELETE) and cannot be navigated further.
+ */
+@FunctionalInterface
+public interface ElideNavigatorOnRelationshipLink {
+  String build();
+}

api/src/test/java/com/faforever/commons/api/elide/ElideNavigatorTest.java

@@ -105,6 +105,14 @@ void testRelationshipLinkAfterNavigateRelationship() {
                              .build(), is("/data/mapPool/5/mapVersion/1234/relationships/map"));
   }
 
+  @Test
+  void testCannotRelationshipLinkAfterIncludes() {
+    assertThrows(IllegalStateException.class, () -> ElideNavigator.of(MapPool.class)
+                                                                  .id("5")
+                                                                  .addInclude("mapVersion")
+                                                                  .relationshipLink("mapVersion"));
+  }
+
   @Test
   void testGetListPages() {
     assertThat(ElideNavigator.of(MapPoolAssignment.class)