fix(ci): carry the failing spec's error into the e2e job summary (refs #161)

The per-spec table now has a "Why" column holding the spec's first error,
flattened for a markdown cell: ANSI stripped, newlines collapsed, `|` escaped
(a real report's message is multi-line, coloured, and embeds source-snippet
gutters), clipped to 300 chars. The column only appears when something failed.

So a red e2e names its cause in the summary even when the log is truncated or
the run is killed mid-stream — which is the state #161 was filed from.

Shape verified against an actual @playwright/test 1.61 failing report, not
just the fixture.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

infra/test_playwright_summary.py

@@ -62,6 +62,26 @@ def test_failing_spec_reports_why():
     assert not any(line.startswith("Call log:") for line in md.splitlines()), md
 
 
+def test_real_playwright_error_is_flattened():
+    # A real report's message is multi-line and ANSI-coloured, and embeds the source snippet with
+    # `|` gutters — all three would break the table cell. Shape verified against an actual
+    # @playwright/test 1.61 JSON report.
+    md = render({
+        "stats": {"expected": 0, "unexpected": 1, "flaky": 0, "skipped": 0, "duration": 1_000},
+        "suites": [{"file": "catalogus.spec.ts", "specs": [
+            spec_entry("a beheerder sees the catalogus", "unexpected",
+                       ["Error: expect(locator).toBeVisible() failed\n\n"
+                        "\x1b[2mLocator: \x1b[22mgetByRole('heading')\n"
+                        "  12 |   await login(page);\n> 13 |   await expect(heading).toBeVisible();\n"]),
+        ]}],
+    })
+    row = [line for line in md.splitlines() if line.startswith("| catalogus.spec.ts")][0]
+    assert "\x1b" not in row, row
+    assert "Locator: getByRole('heading')" in row, row
+    # Every literal `|` from the snippet gutters is escaped, so the row keeps exactly 3 cells.
+    assert row.count("|") - row.count("\\|") == 4, row
+
+
 def test_passing_run_stays_quiet():
     md = render({
         "stats": {"expected": 1, "unexpected": 0, "flaky": 0, "skipped": 0, "duration": 5_000},