perf(protocol): skip string debug escaping when proto debug is off

output_escape() was called on every send_str/receive_str even when
LOG_DEBUG_PROTO logging was disabled, allocating and scanning the whole
payload for a line that log_debug_message() then discarded.  Add a
log_debug_enabled(flag) gate mirroring log_debug_message()'s own filter and
check it before escaping.  Redacted (secret) strings still log the same
<redacted> marker; no observable log output changes.

src/shared/log.c

@@ -27,6 +27,10 @@ uint32_t get_log_debug_flags(void) {
   return current_debug_flags;
 }
 
+bool log_debug_enabled(LogDebugFlag flag) {
+  return current_log_level <= LOG_LEVEL_DEBUG && (current_debug_flags & flag) != 0;
+}
+
 void set_log_info_flags(uint32_t flags) {
   info_flags = flags;
   info_flags_explicit = true;