fix(report): prevent code-fence breakout in vulnerability markdown (#817)

* fix(report): prevent code-fence breakout in vulnerability markdown

render_vulnerability_md wrapped LLM-authored poc_script_code and code
snippet values in a fixed three-backtick fence, so a triple-backtick inside
the value closed the fence early and the rest rendered as live markdown
(headings, tracking-beacon images) in the shareable report deliverable.

Open each such block with a fence one backtick longer than the longest
backtick run in the payload (CommonMark: a block closes only on a fence at
least as long as the opener), so the content always renders verbatim. The
adjacent ```diff block is already safe (its lines are '- '/'+ ' prefixed and
so can never be a bare-backtick closing fence) and is left unchanged.

Fixes #815

* fix(report): indent multiline snippets

---------

Co-authored-by: thejesh23 <thejesh23@users.noreply.github.com>
Co-authored-by: Alex Schapiro <bearsyankees@gmail.com>

tests/test_report_writer.py

@@ -113,6 +113,28 @@ def test_render_vulnerability_md_includes_dependency_fields() -> None:
     assert "## Assumptions" in md
 
 
+def test_render_vulnerability_md_poc_code_cannot_break_out_of_fence() -> None:
+    # LLM/target-authored PoC content containing its own ``` must not close the
+    # fence early and turn the injected markdown into live headings/images.
+    injected = "curl x\n```\n\n## Injected Heading\n![x](https://evil.example/beacon.png)"
+    md = render_vulnerability_md(_sample_report(poc_script_code=injected))
+    lines = md.split("\n")
+    fence = next(ln for ln in lines[lines.index("## Proof of Concept") + 1 :] if ln.strip())
+    assert set(fence) == {"`"}
+    assert len(fence) >= 4  # wider than the payload's 3-backtick run
+    assert injected in md  # the payload survives verbatim, inside the fence
+
+
+def test_render_vulnerability_md_snippet_cannot_break_out_of_fence() -> None:
+    snippet = "row = q()\n```\n## Injected"
+    md = render_vulnerability_md(
+        _sample_report(code_locations=[{"file": "app.py", "snippet": snippet}]),
+    )
+    assert (
+        "  ````\n  row = q()\n  ```\n  ## Injected\n  ````"
+    ) in md  # indented fence widened past the payload's ``` run
+
+
 def test_write_vulnerabilities_creates_markdown_csv_and_json(tmp_path: Path) -> None:
     reports = [
         _sample_report(id="vuln-0001", severity="medium", timestamp="2026-07-02 11:00:00 UTC"),