refactor(test): drop double-cast on Person fixtures

Drops the `as unknown as Person` double-cast in makePerson and on
AUGUSTE/ANNA in favor of plain return-typed object literals; this
restores the type-system safety net Felix flagged on PR #629 — a
future required field on Person now fails compilation in the fixture
instead of silently slipping through.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/MentionDropdown.svelte.test.ts

@@ -10,18 +10,18 @@ type Person = components['schemas']['Person'];
 
 afterEach(cleanup);
 
-const makePerson = (id: string, name: string, overrides: Partial<Person> = {}): Person =>
-	({
+const makePerson = (id: string, name: string, overrides: Partial<Person> = {}): Person => {
+	const parts = name.split(' ');
+	return {
 		id,
-		firstName: name.split(' ')[0] ?? null,
-		lastName: name.split(' ').slice(1).join(' ') || name,
+		firstName: parts[0],
+		lastName: parts.slice(1).join(' ') || name,
 		displayName: name,
 		personType: 'PERSON',
 		familyMember: false,
-		birthYear: null,
-		deathYear: null,
 		...overrides
-	}) as unknown as Person;
+	};
+};
 
 type DropdownState = {
 	items: Person[];