fix: never write past a JSON buffer or send a broken socket event

When something didn't fit, the formatters skipped it silently, closing
brackets included, and clients got malformed JSON.  Two writes could
also go past the buffer:

- AsyncJsonResp flushed when full and then wrote the value without
  checking again, so a value bigger than the whole buffer overflowed.
  It is now streamed straight out instead.
- JsonSockEvent::closeEvent added the closing ] when only one byte was
  left, putting the terminator one byte past the buffer.

The formatters now set an overflowed() flag and log an error, and a
socket event that overflowed is logged and not sent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/JsonFormatter.cpp

@@ -1,10 +1,14 @@
 #include "JsonFormatter.h"
+#include "esp_log.h"
+
+static const char *TAG = "JSON";
 
 void BufferedJsonFormatter::setBuffer(char *b, size_t sz) {
   this->buff = b;
   this->buffSize = sz;
   this->buff[0] = 0;
   this->_nocomma = true;
+  this->_overflow = false;
   this->_objects = 0;
   this->_arrays = 0;
 }
@@ -86,6 +90,8 @@ void JsonFormatter::_safecat(const char *val, bool escape) {
   size_t len = (escape ? this->calcEscapedLength(val) : strlen(val)) + strlen(this->buff);
   if(escape) len += 2;
   if(len >= this->buffSize) {
+    if(!this->_overflow) ESP_LOGE(TAG, "JSON exceeded buffer size %u", (unsigned)this->buffSize);
+    this->_overflow = true;
     return;
   }
   if(escape) strcat(this->buff, "\"");