Fix List<T> response body return types (#79)

Because List<Foo>.class is not valid, we need to use an
Arrays.asList(...) intermediary.

Co-authored-by: (Jonathan) Luke Green <lukeg496@gmail.com>
Co-authored-by: Hugo Pereira <hdcpereira@protonmail.com>

modules/lily-compiler/src/main/java/io/github/tomboyo/lily/compiler/ast/Fqn.java

@@ -4,6 +4,7 @@
 
 import java.nio.file.Path;
 import java.util.List;
+import java.util.Optional;
 import java.util.stream.Collectors;
 
 /**
@@ -84,6 +85,19 @@ public PackageName toPackage() {
     return packageName.resolve(typeName);
   }
 
+  /**
+   * If this FQN describes a List, return an Optional of the List element's FQN, or else an empty
+   * optional.
+   */
+  public Optional<Fqn> listType() {
+    if (packageName.toString().equalsIgnoreCase("java.util")
+        && typeName.toString().equalsIgnoreCase("List")) {
+      return Optional.of(typeParameters.get(0));
+    } else {
+      return Optional.empty();
+    }
+  }
+
   public static class Builder {
     private PackageName packageName = PackageName.of("java.lang");
     private SimpleName typeName;

modules/lily-compiler/src/main/java/io/github/tomboyo/lily/compiler/cg/AstResponseCodeGen.java

@@ -39,7 +39,7 @@ public java.net.http.HttpResponse<? extends java.io.InputStream> httpResponse()
                 return this.httpResponse;
               }
 
-              {{#contentTypeName}}
+              {{#bodyReturnTypeName}}
               /**
                * Return the deserialized representation of the response body if possible. The body
                * is deserialized lazily; if this method is never called, the body is never
@@ -49,19 +49,34 @@ public java.net.http.HttpResponse<? extends java.io.InputStream> httpResponse()
                * @throws java.io.IOException If the response body cannot be deserialized for any
                *         reason.
                */
-              public {{{contentTypeName}}} body() throws java.io.IOException {
-                return objectMapper.readValue(httpResponse.body(), {{{contentTypeName}}}.class);
+              public {{{bodyReturnTypeName}}} body() throws java.io.IOException {
+                {{! List<Foo>.class is not valid java, so we use an Array type intermediary. }}
+                {{#bodyListReturnTypeName}}
+                  return java.util.Arrays.asList(objectMapper.readValue(httpResponse.body(), {{{bodyListReturnTypeName}}}[].class));
+                {{/bodyListReturnTypeName}}
+                {{^bodyListReturnTypeName}}
+                  return objectMapper.readValue(httpResponse.body(), {{{bodyReturnTypeName}}}.class);
+                {{/bodyListReturnTypeName}}
               }
-              {{/contentTypeName}}
+              {{/bodyReturnTypeName}}
             }
             """,
             "renderAstResponse",
             Map.of(
-                "packageName", astResponse.name().packageName(),
-                "typeName", astResponse.name().typeName(),
-                "interfaceName", astResponse.sumTypeName(),
-                "contentTypeName",
-                    astResponse.contentName().<Object>map(Fqn::toFqpString).orElse(false)));
+                "packageName",
+                astResponse.name().packageName(),
+                "typeName",
+                astResponse.name().typeName(),
+                "interfaceName",
+                astResponse.sumTypeName(),
+                "bodyReturnTypeName",
+                astResponse.contentName().<Object>map(Fqn::toFqpString).orElse(false),
+                "bodyListReturnTypeName",
+                astResponse
+                    .contentName()
+                    .flatMap(Fqn::listType)
+                    .<Object>map(Fqn::toFqpString)
+                    .orElse(false)));
 
     return new Source(astResponse.name(), content);
   }

modules/lily-compiler/src/test/java/io/github/tomboyo/lily/compiler/CandlepinApiTest.java

@@ -4,12 +4,10 @@
 
 import io.github.tomboyo.lily.compiler.LilyExtension.LilyTestSupport;
 import java.net.URI;
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 
 /** Compiles an open-source OpenAPI document for a real system to help unearth bugs. */
-@Disabled("Will not pass until '#74 - Graceful Failure' is finished.")
 @ExtendWith(LilyExtension.class)
 class CandlepinApiTest {
   @Test