[core] Replace strnlen in buf_append_str for Zephyr compatibility (#15892)

esphome/core/helpers.h

@@ -1117,7 +1117,10 @@ inline size_t buf_append_str(char *buf, size_t size, size_t pos, const char *str
     return size;
   }
   size_t remaining = size - pos - 1;  // reserve space for null terminator
-  size_t len = strnlen(str, remaining);
+  size_t len = 0;
+  while (len < remaining && str[len] != '\0') {
+    len++;
+  }
   memcpy(buf + pos, str, len);
   pos += len;
   buf[pos] = '\0';