fix(ocr): sanitize Graph validationToken echo to prevent reflected XSS

CodeQL js/reflected-xss (high) flagged AI/ocr/server/onReceiptAdded.ts:
the Microsoft Graph subscription validationToken from req.query was echoed
straight into the HTTP response body. Although the response is sent as
text/plain, a user-controlled value reflected verbatim is a reflected XSS
sink (browsers can MIME-sniff, and the value is attacker influenceable).

Graph requires the opaque, URL-safe validationToken to be echoed back to
complete the subscription handshake, so strip any character outside the
token's known-safe set (base64url/base64) before reflecting it. This is a
no-op for legitimate tokens while removing the characters needed for XSS.
Also set X-Content-Type-Options: nosniff as defense in depth.

Verified on Windows (Node 24): backend builds; validate-sample.ps1 backend
smoke passes; a legitimate token echoes verbatim (handshake intact) while
'<script>alert(1)</script>' is returned as 'scriptalert1/script' (angle
brackets stripped).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Greg Joseph

AI/ocr/server/onReceiptAdded.ts

@@ -6,10 +6,23 @@ import { ReceiptProcessor } from "./ReceiptProcessor";
 
 require('isomorphic-fetch');
 
+  // Microsoft Graph sends an opaque, URL-safe validationToken when a subscription is
+  // created and expects it echoed back verbatim as text/plain. The value is attacker
+  // influenceable, so strip anything outside the token's known-safe character set
+  // before reflecting it. This is a no-op for legitimate tokens (base64url/base64)
+  // while removing the characters needed for reflected cross-site scripting.
+  const sanitizeValidationToken = (value: unknown): string =>
+    String(value).replace(/[^A-Za-z0-9._~+/=\- ]/g, '');
+
   export const onReceiptAdded = async (req: Request, res: Response) => {
     const validationToken = req.query['validationToken'];
     if (validationToken) {
-      res.status(200).type('text/plain').send(String(validationToken));
+      const safeValidationToken = sanitizeValidationToken(validationToken);
+      res
+        .status(200)
+        .type('text/plain')
+        .set('X-Content-Type-Options', 'nosniff')
+        .send(safeValidationToken);
         return;
     }