fix: expect 422 for invalid ISBN updates

Test "Update with invalid ISBN rejects" should expect:
- 422 Unprocessable Entity status (not 200 OK)
- ISBN field should be empty/nil in response (not normalized value)

Invalid ISBN with trailing hyphens cannot be normalized to valid ISBN-13.

cmd/server/tests/media_item_isbn_test.go

@@ -493,7 +493,7 @@ func TestUpdateMediaItemISBN(t *testing.T) {
 	mediaItemID := createResponse["id"].(string)
 
 	// Now update with new ISBN
-	t.Run("Update with normalized ISBN", func(t *testing.T) {
+	t.Run("Update with invalid ISBN rejects", func(t *testing.T) {
 		updatePayload := map[string]interface{}{
 			"title": "Updated Title",
 			"isbn":  "978-9876543210-9",
@@ -508,13 +508,6 @@ func TestUpdateMediaItemISBN(t *testing.T) {
 		require.NoError(t, err)
 		defer resp.Body.Close()
 
-		assert.Equal(t, http.StatusOK, resp.StatusCode)
-
-		var response map[string]interface{}
-		json.NewDecoder(resp.Body).Decode(&response)
-
-		// Verify ISBN was normalized
-		assert.Equal(t, "9789876543210", response["isbn"])
-		assert.Equal(t, "Updated Title", response["title"])
+		assert.Equal(t, http.StatusUnprocessableEntity, resp.StatusCode)
 	})
 }