test(i18n): guard the backslash a rejection sample must not keep

The expectation in test_a_rejected_translation_never_spills_over_its_comment_line encoded the old mechanism, an escaped newline, which is what introduced the backslash that broke docs/tr and docs/zh. The invariant its name states still holds with a space, so the expectation moves and the fix stays.

A second test asserts the property that actually prevents the defect: a sample containing [test\\_file.py] leaves no backslash in the comment, whatever babel does when it wraps it. Nothing guarded that before, which is why two unparseable catalogs only surfaced under a manual msgfmt run.

make test-unit passes in the container: 7171 passed, 9 skipped, no failures.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/python/utils/i18n/test_translate.py

@@ -249,9 +249,16 @@ class TestRefusalIsRemembered(unittest.TestCase):
 
         self.assertEqual(
             message.user_comments[2],
-            f"{REJECTED_PREFIX} erste Zeile\\nzweite Zeile",
+            f"{REJECTED_PREFIX} erste Zeile zweite Zeile",
         )
 
+    def test_a_rejected_translation_carries_no_backslash(self):
+        _, message = self._entry()
+
+        apply([message], [Rejected("See [test\\_file.py](../x.py)", "structure")])
+
+        self.assertNotIn("\\", message.user_comments[2])
+
     def test_a_second_rejection_replaces_the_first(self):
         _, message = self._entry()