fix(6255): Test destination names an address that did not answer as S3

A 4xx with no S3 error body came from something else at that address, most
often a web server on the default port. The message now says so and to check
the scheme and port, beside the raw status. A 5xx without a body keeps its
own reason, since a busy S3 service can answer that way.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/grimoire/services/backup_s3.py

@@ -120,6 +120,12 @@ def _says(response: httpx.Response) -> str:
         "AccessDenied": "the key isn't allowed to do that",
     }
     said = hints.get(code) or message or response.reason_phrase
+    if not code and 400 <= response.status_code < 500:
+        # S3 refuses with an <Error><Code> body. A 4xx without one came from
+        # something else at that address — most often a web server on the
+        # default port, when the S3 service listens on a port of its own.
+        said = ("the address answered, but not as S3 — check the scheme and "
+                "the port (an S3 service usually listens on its own port)")
     return f"HTTP {response.status_code}{f' {code}' if code else ''}: {said}."