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_issue_code_syntax_highlight.py

@@ -17,9 +17,10 @@ def test_fenced_code_blocks_add_prism_language_class():
 def test_fenced_code_blocks_keep_existing_pre_header_layout():
     js = _read_ui_js()
     # The fenced code rendering was moved into the stash callback (#1154 fix).
-    # The template string now uses `lang` instead of `normalizedLang`.
-    assert '${h}<pre><code${langAttr}>${esc(code.replace(/\\n$/,' in js, (
-        "The syntax-highlight fix should preserve the existing fenced code block layout"
+    # The template string now uses `lang` instead of `normalizedLang`, and
+    # markdown-source fences may add a dedicated md-source-block class.
+    assert '${h}<pre${preClass}><code${langAttr}>${esc(code.replace(/\\n$/,' in js, (
+        "The syntax-highlight fix should preserve the shared fenced code block layout"
     )
     assert '<div class="code-block">' not in js, (
         "This fix should not introduce a new wrapper around fenced code blocks"