gh-127794: Validate email header names according to RFC 5322 (#127820)

`email.message.Message` objects now validate header names specified via `__setitem__`
or `add_header` according to RFC 5322, §2.2 [1].

In particular, callers should expect a ValueError to be raised for invalid header names.

[1]: https://datatracker.ietf.org/doc/html/rfc5322#section-2.2

---------

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: R. David Murray <rdmurray@bitdance.com>

Lib/test/test_email/test_message.py

@@ -1004,6 +1004,30 @@ def test_folding_with_long_nospace_http_policy_1(self):
         parsed_msg = message_from_bytes(m.as_bytes(), policy=policy.default)
         self.assertEqual(parsed_msg['Message-ID'], m['Message-ID'])
 
+    def test_invalid_header_names(self):
+        invalid_headers = [
+            ('Invalid Header', 'contains space'),
+            ('Tab\tHeader', 'contains tab'),
+            ('Colon:Header', 'contains colon'),
+            ('', 'Empty name'),
+            (' LeadingSpace', 'starts with space'),
+            ('TrailingSpace ', 'ends with space'),
+            ('Header\x7F', 'Non-ASCII character'),
+            ('Header\x80', 'Extended ASCII'),
+        ]
+        for email_policy in (policy.default, policy.compat32):
+            for setter in (EmailMessage.__setitem__, EmailMessage.add_header):
+                for name, value in invalid_headers:
+                    self.do_test_invalid_header_names(email_policy, setter, name, value)
+
+    def do_test_invalid_header_names(self, policy, setter, name, value):
+        with self.subTest(policy=policy, setter=setter, name=name, value=value):
+            message = EmailMessage(policy=policy)
+            pattern = r'(?i)(?=.*invalid)(?=.*header)(?=.*name)'
+            with self.assertRaisesRegex(ValueError, pattern) as cm:
+                 setter(message, name, value)
+            self.assertIn(f"{name!r}", str(cm.exception))
+
     def test_get_body_malformed(self):
         """test for bpo-42892"""
         msg = textwrap.dedent("""\