fix(desktop): wire serializeJsonBody into OAuth request path

The salvaged helper exported serializeJsonBody but main.cjs still inline-built
the request body, leaving the export dead and the test decoupled from the real
path. Use it at the fetchJsonViaOauthSession site so the helper's coverage
exercises production body construction. Byte-identical output.

apps/desktop/electron/main.cjs

@@ -3468,7 +3468,7 @@ function fetchJsonViaOauthSession(url, options = {}) {
       reject(new Error(`Unsupported Hermes backend URL protocol: ${parsed.protocol}`))
       return
     }
-    const body = options.body === undefined ? undefined : Buffer.from(JSON.stringify(options.body))
+    const body = serializeJsonBody(options.body)
     const timeoutMs = resolveTimeoutMs(options.timeoutMs, DEFAULT_FETCH_TIMEOUT_MS)
 
     const request = electronNet.request({