car_diff: dedupe errors and render them in a collapsible dropdown

A single failing carcontroller raised the same traceback for every
segment, printing dozens of identical tracebacks at the top of the
report. Group errors by their text, print each unique one once with the
affected segment count, and wrap the whole block in a details dropdown
like the diff section so it does not spam the PR comment.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dkoep

opendbc/car/tests/car_diff.py

@@ -293,8 +293,16 @@ def main(platform: str | None = None, segments_per_platform: int = 10, update_re
   icon = "⚠️" if with_diffs else "✅"
   print(f"\n{icon}  {len(with_diffs)} changed, {n_passed} passed, {len(errors)} errors")
 
-  for plat, seg, err in errors:
-    print(f"\nERROR {plat} - {seg}: {err}")
+  if errors:
+    # Group identical errors so a single bug doesn't spam the report with one traceback per segment
+    by_err: dict[str, list[str]] = defaultdict(list)
+    for plat, seg, err in errors:
+      by_err[err].append(f"{plat} - {seg}")
+    print("<details><summary><b>Show errors</b></summary>\n\n```")
+    for err, segs in sorted(by_err.items(), key=lambda kv: -len(kv[1])):
+      affected = ", ".join(segs[:3]) + (f" (+{len(segs) - 3} more)" if len(segs) > 3 else "")
+      print(f"\nERROR ({len(segs)}x) {affected}:\n{err.rstrip()}")
+    print("```\n</details>")
 
   if with_diffs:
     print("<details><summary><b>Show changes</b></summary>\n\n```")