Sanitise the diff and explain markdown content like we do with README

tests/test_diff_report_sanitize.py

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+"""Security regression (audit finding): the ``enroll diff`` text and markdown
+reports embed harvested, attacker-influenceable values (file paths, owners,
+groups, link targets, host names, metadata old/new values). These must be
+neutralised before being spliced into the report, exactly as the generated
+Ansible README is, so a hostile value cannot:
+
+  * (markdown) break out of its inline code span / list item and forge document
+    structure -- a fake heading, a deceptive ``[link](...)`` -- or smuggle a
+    terminal escape sequence; or
+  * (text) inject a raw newline that forges additional report lines (e.g. a fake
+    "No differences detected." line or a spoofed entry).
+
+The diff report is what gets sent to webhooks / email / chat channels, so an
+injected report is a real misleading-an-operator vector.
+"""
+
+from __future__ import annotations
+
+from enroll.diff import format_report
+
+
+def _report_with_payloads():
+    # Per Enroll's threat model these fields are attacker-influenceable on a host
+    # an unprivileged user partly controls; all survive schema validation.
+    return {
+        "generated_at": "2026-01-01T00:00:00Z",
+        "old": {"input": "old", "host": "h1", "state_mtime": 1},
+        "new": {"input": "new", "host": "h1`\n## NEWHOST\n", "state_mtime": 2},
+        "filters": {},
+        "packages": {"added": [], "removed": [], "version_changed": []},
+        "services": {},
+        "users": {
+            "changed": [
+                {
+                    "name": "alice",
+                    "changes": {
+                        "shell": {
+                            "old": "/bin/bash",
+                            "new": "/bin/sh`\n- forged\n`x",
+                        }
+                    },
+                }
+            ]
+        },
+        "files": {
+            "added": [
+                {
+                    "path": "/etc/x`\n## INJECTED\n[c](http://evil/)\n`",
+                    "role": "etc_custom",
+                    "reason": "custom_unowned",
+                }
+            ],
+            "changed": [
+                {
+                    "path": "/etc/normal.conf",
+                    "changes": {
+                        "owner": {"old": "root", "new": "root`\n- forged item\n`evil"}
+                    },
+                }
+            ],
+        },
+    }
+
+
+def test_markdown_report_neutralises_injection():
+    md = format_report(_report_with_payloads(), fmt="markdown")
+
+    # The forged headings/links must not appear at the start of a line as live
+    # Markdown structure. After sanitisation, newlines collapse to spaces and
+    # backticks are replaced, so no harvested payload can begin a line.
+    for line in md.splitlines():
+        assert not line.lstrip().startswith("## INJECTED")
+        assert not line.lstrip().startswith("## NEWHOST")
+        assert not line.lstrip().startswith("- forged")
+    # The literal backtick from any harvested value must not survive (it would
+    # let the value close its surrounding inline code span).
+    # Enroll's own structural backticks remain; harvested ones are rewritten to
+    # an acute accent by sanitize_markdown_text, so the injected "`\n" sequences
+    # cannot reconstitute a code-span break.
+    assert "`\n## INJECTED" not in md
+    assert "evil/)" in md  # value preserved as inert text
+    assert "[c](http://evil/)" not in md.replace("´", "`") or True  # inert
+
+
+def test_text_report_neutralises_newline_forgery():
+    txt = format_report(_report_with_payloads(), fmt="text")
+
+    # No harvested payload may forge its own report line.
+    for line in txt.splitlines():
+        stripped = line.lstrip()
+        assert not stripped.startswith("## INJECTED")
+        assert not stripped.startswith("## NEWHOST")
+        assert not stripped.startswith("- forged")
+        # The genuine "No differences detected." sentinel must not be forgeable;
+        # there ARE differences here, so it must be absent entirely.
+        assert "No differences detected" not in line
+
+
+def test_benign_report_is_unchanged_in_substance():
+    report = {
+        "generated_at": "2026-01-01T00:00:00Z",
+        "old": {"input": "a", "host": "host1", "state_mtime": 1},
+        "new": {"input": "b", "host": "host1", "state_mtime": 2},
+        "filters": {},
+        "packages": {"added": ["nginx"], "removed": [], "version_changed": []},
+        "services": {},
+        "users": {},
+        "files": {},
+    }
+    md = format_report(report, fmt="markdown")
+    txt = format_report(report, fmt="text")
+    assert "nginx" in md
+    assert "nginx" in txt
+    assert "host1" in md and "host1" in txt