gh-66335: Test uppercase IMAP4 command names

Co-authored-by: Milan Oberkirch <zvyn@oberkirch.org>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

Lib/test/test_imaplib.py

@@ -1415,6 +1415,16 @@ def test_xatom(self):
         self.assertEqual(data, [b'MYCOMMAND completed'])
         self.assertEqual(server.args, ['arg1', 'arg2'])
 
+    def test_uppercase_command_names(self):
+        client, server = self._setup(SimpleIMAPHandler)
+        client.login('user', 'pass')
+        self.assertEqual(client.CAPABILITY, client.capability)
+        self.assertEqual(client.SELECT, client.select)
+        typ, data = client.CAPABILITY()
+        self.assertEqual(typ, 'OK')
+        with self.assertRaises(AttributeError):
+            client.NONEXISTENT
+
     def test_control_characters(self):
         client, _ = self._setup(SimpleIMAPHandler)
         for c0 in support.control_characters_c0():