refactor(font): move test fixtures into a dedicated testing segment

Mock data lived in lib/ and was re-exported through the slice's
production index.ts, advertising fixtures as part of the public API.
Move it to a testing/ segment and drop it from index.ts; consumers now
import explicitly via the $entities/Font/testing subpath.

Repoints the four mock consumers (FontApplicator story, sizeResolver
and fontCatalogStore specs, comparisonStore test) to the new subpath.

src/entities/Font/model/store/fontCatalogStore/fontCatalogStore.svelte.spec.ts

@@ -1,3 +1,7 @@
+import {
+    generateMixedCategoryFonts,
+    generateMockFonts,
+} from '$entities/Font/testing';
 import { QueryClient } from '@tanstack/query-core';
 import { flushSync } from 'svelte';
 import {
@@ -12,10 +16,6 @@ import {
     FontNetworkError,
     FontResponseError,
 } from '../../../lib/errors/errors';
-import {
-    generateMixedCategoryFonts,
-    generateMockFonts,
-} from '../../../lib/mocks/fonts.mock';
 import type { UnifiedFont } from '../../types';
 import { FontCatalogStore } from './fontCatalogStore.svelte';