Add a unit test

Gemfile.lock

@@ -19,6 +19,7 @@ GEM
     parser (3.3.3.0)
       ast (~> 2.4.1)
       racc
+    power_assert (2.0.3)
     prism (0.30.0)
     puma (6.4.2)
       nio4r (~> 2.0)
@@ -29,6 +30,8 @@ GEM
       rack (>= 3.0.0, < 4)
     rack-session (2.0.0)
       rack (>= 3.0.0)
+    rack-test (2.1.0)
+      rack (>= 1.3)
     rackup (2.1.0)
       rack (>= 3)
       webrick (~> 1.8)
@@ -72,6 +75,8 @@ GEM
       tilt (~> 2.0)
     sorbet-runtime (0.5.11435)
     strscan (3.1.0)
+    test-unit (3.6.2)
+      power_assert
     tilt (2.3.0)
     ulid (1.4.0)
     unicode-display_width (2.5.0)
@@ -90,12 +95,14 @@ DEPENDENCIES
   ksuid
   nanoid
   puma
+  rack-test
   rackup
   rbs
   rubocop
   ruby-lsp
   sinatra
   sinatra-contrib
+  test-unit
   ulid
   uuid7