fix: handle invalid date formats gracefully and add missing dialog descriptions

- Wrap date.fromisoformat() in try/except in case_update tool — prevents
  unhandled ValueError from surfacing as 500; FastAPI now catches it as 422
- Add DialogDescription (sr-only) to 5 dialogs missing aria-describedby:
  documents-panel preview + delete, drafts-panel delete + feedback, link-related-dialog

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mcp-server/src/legal_mcp/tools/cases.py

@@ -326,9 +326,15 @@ async def case_update(
     if notes:
         fields["notes"] = notes
     if hearing_date:
-        fields["hearing_date"] = date_type.fromisoformat(hearing_date)
+        try:
+            fields["hearing_date"] = date_type.fromisoformat(hearing_date)
+        except ValueError as exc:
+            raise ValueError(f"Invalid hearing_date format: {hearing_date!r}") from exc
     if decision_date:
-        fields["decision_date"] = date_type.fromisoformat(decision_date)
+        try:
+            fields["decision_date"] = date_type.fromisoformat(decision_date)
+        except ValueError as exc:
+            raise ValueError(f"Invalid decision_date format: {decision_date!r}") from exc
     if tags is not None:
         fields["tags"] = tags
     if expected_outcome: