Handle cache miss and redirect in SignedPage

Added a check for `_sig` being `null` to handle cache misses or missing `sid`. Logged a warning with `Sid` and `EnvelopeKey` details when this occurs. Implemented a redirection to the report page (`/envelope/{EnvelopeKey}/report`) using `Navigation.NavigateTo` with `forceLoad: true`. Added an early return to prevent further execution after redirection.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeReceiverReportSignedPage.razor

@@ -120,6 +120,18 @@
             _sig = cached;
         }
 
+        // Cache miss or missing sid — redirect back to report page
+        if (_sig is null)
+        {
+            Logger.LogWarning(
+                "[SignedPage] Cache miss or no sid={Sid} for {EnvelopeKey}, redirecting to report page.",
+                Sid, EnvelopeKey);
+            Navigation.NavigateTo(
+                $"/envelope/{Uri.EscapeDataString(EnvelopeKey)}/report",
+                forceLoad: true);
+            return;
+        }
+
         try
         {
             var pdfBytes = await PageDataService.GetDocumentAsync(_receiverUser);