fix(reports): grow the markdown fence past backticks in the message

[Chemaclass]

🤔 Background

Related #1305

--report-md fences each failure message unescaped, because a fence renders its contents literally — true only while the fence is longer than any run of backticks inside it. A hook printing a bare fence closed the block early, so the text after it rendered as prose and the trailing fence opened a new, unterminated one. Every later section of the report was swallowed.

That report is auto-appended to $GITHUB_STEP_SUMMARY, so it corrupts the Actions job summary — the page the report exists to be read on.

💡 Changes

• The opening fence grows past the longest run of backticks present, which is how CommonMark closes a fence. Fork-free and terminating: the message is finite, and testing containment rather than equality finds the longest run, since a message holding four backticks contains three too
• The regression test asserts the property, not the implementation: the longest backtick-only line is the delimiter, and it must appear an even number of times. Counting every backtick-only line would call the fixed document broken, since the message keeps its own inner fence

🔍 Notes

The diff-rendered case survived by luck — assert_same wraps changed lines as [-...-], which is not a bare delimiter. Undiffed messages (hook failures, source failures) reach the report intact, which is why this showed up in a hook fixture.

HTML was swept at the same time and is clean: <script> is escaped and there are no stray ampersands. The Markdown summary table has no message column, so no | injection either.