stage #4669 (TomBanksAU): wrap markdown source previews — AUTHORITATIVE head 0cee92c9 (#4679)

Re-staged from the contributor's rebased head (0cee92c9) which independently converged on
the same 5-test-file class-tolerance fixups I'd made — shipping the contributor's exact
version, not my private variant. Tags md/markdown/mdx fenced blocks with md-source-block
class in both renderers; CSS wraps them while code/diff/csv keep horizontal-scroll. 90
renderer-family tests pass.

Co-authored-by: nesquena-hermes <agent@nesquena-hermes>
Co-authored-by: TomBanksAU <TomBanksAU@users.noreply.github.com>

tests/test_1325_user_fenced_code.py

@@ -126,7 +126,7 @@ class TestUserFencedBlocks:
     def test_four_backtick_outer_fence_preserves_inner_triple_fence(self):
         """User-message code fences should follow CommonMark fence-length matching too."""
         out = _run_user_render("````md\n```inner\nfoo\n```\n````")
-        assert out.count("<pre>") == 1
+        assert out.count("<pre class=\"md-source-block\">") == 1
         assert out.count("</pre>") == 1
         assert '<div class="pre-header">md</div>' in out
         assert "```inner" in out