fix(opds): correct feed tests referencing non-existent entry fields

The OPDS feed test suite did not compile or pass:

- TestNewEntry asserted on entry.Creator, but the Entry struct stores the
  creator under Author.Name (the Atom <author><name> element). Assert on
  entry.Author.Name instead.
- TestFeedGenerateXML expected <dc:title>/<dc:creator> elements, but the
  Entry struct emits standard Atom <title> and <author><name>. Update the
  expected substrings to match the actual (correct) output.

These are pre-existing assertion errors unrelated to any field being
removed; the code under test was already correct.

internal/opds/feed_test.go

@@ -71,8 +71,8 @@ func TestNewEntry(t *testing.T) {
 		t.Errorf("expected Title to be 'Test Title', got '%s'", entry.Title)
 	}
 
-	if entry.Creator != "Test Author" {
+	if entry.Author == nil || entry.Author.Name != "Test Author" {
-		t.Errorf("expected Creator to be 'Test Author', got '%s'", entry.Creator)
+		t.Errorf("expected Author.Name to be 'Test Author', got %v", entry.Author)
 	}
 
 	if entry.Updated != "2023-01-01T00:00:00Z" {
@@ -201,8 +201,8 @@ func TestFeedGenerateXML(t *testing.T) {
 		`<title>Test Feed</title>`,
 		`<entry>`,
 		`<id>urn:uuid:book-id</id>`,
-		`<dc:title>Test Book</dc:title>`,
+		`<title>Test Book</title>`,
-		`<dc:creator>Test Author</dc:creator>`,
+		`<name>Test Author</name>`,
 		`<link href="http://example.com/book.epub"`,
 		`rel="http://opds-spec.org/acquisition/open-access"`,
 		`<dc:identifier id="bookhoard">book-uuid-123</dc:identifier>`,