fix: ble_scanner sha256_prefix — guard mbedTLS null info and setup failure

firmware/src/ble_scanner.cpp

@@ -20,15 +20,21 @@ static std::map<String, DeviceObs> s_seen;
 static int s_max_concurrent = 0;
 
 static String sha256_prefix(const String& input) {
-    uint8_t hash[32];
-    mbedtls_md_context_t ctx;
     const mbedtls_md_info_t* info = mbedtls_md_info_from_type(MBEDTLS_MD_SHA256);
+    if (!info) return String(); // SHA256 not available
+
+    uint8_t hash[32] = {};
+    mbedtls_md_context_t ctx;
     mbedtls_md_init(&ctx);
-    mbedtls_md_setup(&ctx, info, 0);
+    if (mbedtls_md_setup(&ctx, info, 0) != 0) {
+        mbedtls_md_free(&ctx);
+        return String();
+    }
     mbedtls_md_starts(&ctx);
     mbedtls_md_update(&ctx, (const uint8_t*)input.c_str(), input.length());
     mbedtls_md_finish(&ctx, hash);
     mbedtls_md_free(&ctx);
+
     String hex = "";
     char buf[3];
     for (int i = 0; i < 16; i++) { snprintf(buf, 3, "%02x", hash[i]); hex += buf; }