refactor(tests): update test helpers and all test file references

- Rename createTestEbookID to createTestMediaItemID in test_helpers.go
- Update test helper comments and variable names (ebookReq -> mediaItemReq, etc.)
- Update all test file references:
  - analytics_test.go
  - book_matching_test.go
  - collections_bulk_test.go
  - kobo_test.go
  - media_bulk_test.go
  - opds_test.go
- Rename ebookID variable to mediaItemID in kobo_test.go
- Update test data to use 'Test Media Item' instead of 'Test Ebook'

cmd/server/tests/analytics_test.go

@@ -331,7 +331,7 @@ func TestAnalyticsPopularBooks(t *testing.T) {
 		token := loginTestUser(t, ts, db)
 
 		// First create a book and some reading history
-		bookID := createTestEbookID(t, ts, token)
+		bookID := createTestMediaItemID(t, ts, token)
 
 		// Create reading history for the book
 		historyReq := map[string]interface{}{