skip tests whose optional dependency is not installed

With pywikidiff2 and mediawiki-utilities no longer installed by default,
the tests covering --diff, -p wikidiff2, and -p legacy cannot run on a
base install. Mark them so a base install reports skips rather than
failures, and keep the markers in wikiq_test_utils.py so all three test
modules share one definition of what each feature needs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
(cherry picked from commit 59d5a9d46a04320bad5d81e0edf5ecd11f9c2a9b)

test/test_wiki_diff_matcher.py

@@ -7,6 +7,10 @@ from typing import List
 from deltas import Delete, Equal, Insert, wikitext_split
 from mwpersistence import Token
 from wikiq.wiki_diff_matcher import WikiDiffMatcher
+from wikiq_test_utils import requires_pywikidiff2
+
+# every test here drives wikidiff2 directly
+pytestmark = requires_pywikidiff2
 
 def _replace_whitespace(match):
     if match.group(1):  # If spaces matched (e.g., '  ')