feat(tests): reopen rejected test keeps all content + rejection notes

Backend (reopen_test):
- Preserve red/blue validation NOTES — teams see exactly what to fix
  without losing the rejection context. Previously both notes were cleared.
- Preserve all content fields: procedure_text, tool_used, red_summary,
  attack_success, blue_summary, detection_result (already the case).
- Preserve evidences (separate table, unaffected — already the case).
- Still clear: validation statuses + who/when validated (fresh re-validation
  required). Phase timing reset so the new execution starts clean.

Frontend:
- Button label: 'Reopen Test' → 'Continue Test' (more accurate intent)
- Dialog title: 'Reopen Test' → 'Continue Test'
- Dialog message: replaces alarming 'workflow will be restarted / clear all'
  with accurate description of what is preserved vs reset
- Toast: explains what to do next

frontend/src/components/test-detail/TestDetailHeader.tsx

@@ -225,7 +225,7 @@ export default function TestDetailHeader({
           className="flex items-center gap-1.5 rounded-lg border border-cyan-500/30 bg-cyan-900/20 px-4 py-2 text-sm font-medium text-cyan-400 hover:bg-cyan-900/40 transition-colors disabled:opacity-50"
         >
           {isTransitioning ? <Loader2 className="h-4 w-4 animate-spin" /> : <RotateCcw className="h-4 w-4" />}
-          Reopen Test
+          Continue Test
         </button>,
       );
     }