docs(ses_auth): note CFWS ends abutting tokens

The space inserted for a removed comment means header.d=domain(note)
now reads as "domain" where the old gluing behaviour produced
"domainnote" (rejected). Document in _strip_comments that this
loosening is deliberate and safe -- header.d comes verbatim from a
verified signature's d= tag, so a comment in that position is never
attacker-controlled -- and pin the behaviour with a regression test.

Review follow-up to PR #103; no runtime behaviour change.

lambdas/po/email_processor/ses_auth.py

@@ -104,6 +104,13 @@ def _strip_comments(value: str) -> tuple:
     outside any comment (depth 0) is an unmatched close and makes the value
     not well-formed, closing the ``)(`` clause-injection gap.
 
+    CFWS also means a comment abutting a token *ends* that token:
+    ``header.d=seahaven.com(note)`` reads as domain ``seahaven.com``, where
+    the pre-CFWS gluing behaviour read ``seahaven.comnote`` (rejected).
+    This is deliberate -- ``header.d`` is written verbatim by SES from a
+    *verified* signature's ``d=`` tag, so a comment in that position is
+    never attacker-controlled.
+
     Returns ``(stripped_text, well_formed)``. ``well_formed`` is False when
     the value ends inside an unterminated comment or quoted-string, or when
     an unmatched ``)`` appears at the top level. Callers reject on

lambdas/wo/email_processor/ses_auth.py

@@ -104,6 +104,13 @@ def _strip_comments(value: str) -> tuple:
     outside any comment (depth 0) is an unmatched close and makes the value
     not well-formed, closing the ``)(`` clause-injection gap.
 
+    CFWS also means a comment abutting a token *ends* that token:
+    ``header.d=seahaven.com(note)`` reads as domain ``seahaven.com``, where
+    the pre-CFWS gluing behaviour read ``seahaven.comnote`` (rejected).
+    This is deliberate -- ``header.d`` is written verbatim by SES from a
+    *verified* signature's ``d=`` tag, so a comment in that position is
+    never attacker-controlled.
+
     Returns ``(stripped_text, well_formed)``. ``well_formed`` is False when
     the value ends inside an unterminated comment or quoted-string, or when
     an unmatched ``)`` appears at the top level. Callers reject on

tests/test_ses_auth.py

@@ -258,6 +258,17 @@ class TestParseAuthenticationResults:
         )
         assert passing == frozenset()
 
+    def test_comment_after_header_d_ends_the_domain_token(self, ses_auth):
+        # CFWS: a comment abutting header.d ends the token, so the domain
+        # reads as "seahaven.com" (pre-CFWS gluing read "seahaven.comnote",
+        # rejected). Deliberate: header.d is written verbatim by SES from a
+        # *verified* signature's d= tag, so a comment in that position is
+        # never attacker-controlled.
+        _, passing = ses_auth.parse_authentication_results(
+            "amazonses.com; dkim=pass header.d=seahaven.com(note)"
+        )
+        assert passing == frozenset({"seahaven.com"})
+
     def test_unbalanced_quote_fails_closed(self, ses_auth):
         # An unterminated quoted-string is malformed; refuse to parse it so a
         # dkim=pass clause "swallowed" by the runaway quote can't be salvaged