[refactor] engines: use datetime.fromisoformat instead of datetime.strptime where possible (#6394)

Refactor engines that parse ISO 8601 dates with strptime to use
fromisoformat instead. In most cases this is a direct replacement of
strptime(text, "format") with fromisoformat(text).

For engines where the source has a trailing "Z" that strptime consumed
as a literal (e.g. "%Y-%m-%dT%H:%M:%S.%fZ" in huggingface.py), add
rstrip("Z") to keep the output naive and preserve the existing behavior.

In sogou.py the date is extracted with a regular expression, which can
yield strings like "2026-7-11". strptime accepts this via its format
string, but fromisoformat does not. To preserve the existing behavior
and satisfy the format fromisoformat expects, add zero-padding for the
month and day.

Closes: #6098
---------
Signed-off-by: OneVth <onebrotravel@gmail.com>

searx/engines/wallhaven.py

@@ -79,7 +79,7 @@ def response(resp):
                 'img_src': result['path'],
                 'thumbnail_src': result['thumbs']['small'],
                 'resolution': result['resolution'].replace('x', ' x '),
-                'publishedDate': datetime.strptime(result['created_at'], '%Y-%m-%d %H:%M:%S'),
+                'publishedDate': datetime.fromisoformat(result['created_at']),
                 'img_format': result['file_type'],
                 'filesize': humanize_bytes(result['file_size']),
             }