test: cover check_em_dash failure and skip paths

tests/test_check_em_dash.py

@@ -1,6 +1,9 @@
 """The em-dash check scans every tracked prose-like file type, not only Markdown and code (#1020)."""
 
 import importlib
+import subprocess
+
+import pytest
 
 check_em_dash = importlib.import_module("scripts.check_em_dash")
 
@@ -15,3 +18,54 @@ def test_tracked_notebooks_and_requirements_are_scanned_and_clean():
     assert "requirements.txt" in scanned
     assert any(path.endswith(".ipynb") for path in scanned)
     assert check_em_dash.main() == 0
+
+
+@pytest.fixture
+def temp_git_repo(tmp_path, monkeypatch):
+    subprocess.run(["git", "init", "-q"], cwd=tmp_path, check=True)
+    monkeypatch.chdir(tmp_path)
+    return tmp_path
+
+
+def test_tracked_em_dash_reports_path_line_and_ignores_untracked_file(
+    temp_git_repo, capsys
+):
+    tracked = temp_git_repo / "notes.md"
+    tracked.write_text("clean line\nfound an \u2014 em dash\n", encoding="utf-8")
+    (temp_git_repo / "untracked.md").write_text(
+        "untracked \u2014 text\n", encoding="utf-8"
+    )
+    subprocess.run(["git", "add", "notes.md"], cwd=temp_git_repo, check=True)
+
+    assert check_em_dash.main() == 1
+
+    output = capsys.readouterr().err
+    assert "notes.md:2:10: found an \u2014 em dash" in output
+    assert "1 em dash(es) across 1 file(s)." in output
+    assert "untracked.md" not in output
+
+
+def test_allowlisted_tracked_path_passes(temp_git_repo, capsys):
+    (temp_git_repo / "CONTRIBUTING.md").write_text(
+        "rule example: \u2014\n", encoding="utf-8"
+    )
+    subprocess.run(["git", "add", "CONTRIBUTING.md"], cwd=temp_git_repo, check=True)
+
+    assert list(check_em_dash._tracked_files()) == []
+    assert check_em_dash.main() == 0
+    assert (
+        capsys.readouterr().out == "OK: no em dashes in tracked source/prose files.\n"
+    )
+
+
+def test_tracked_binary_file_is_skipped(temp_git_repo, capsys):
+    (temp_git_repo / "binary.md").write_bytes(
+        b"binary with em dash: \xe2\x80\x94 and invalid byte: \xff\n"
+    )
+    subprocess.run(["git", "add", "binary.md"], cwd=temp_git_repo, check=True)
+
+    assert list(check_em_dash._tracked_files()) == ["binary.md"]
+    assert check_em_dash.main() == 0
+    assert (
+        capsys.readouterr().out == "OK: no em dashes in tracked source/prose files.\n"
+    )