[uart] Change available() return type from int to size_t (#13893)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

esphome/components/uart/uart_component.cpp

@@ -5,13 +5,13 @@ namespace esphome::uart {
 static const char *const TAG = "uart";
 
 bool UARTComponent::check_read_timeout_(size_t len) {
-  if (this->available() >= int(len))
+  if (this->available() >= len)
     return true;
 
   uint32_t start_time = millis();
-  while (this->available() < int(len)) {
+  while (this->available() < len) {
     if (millis() - start_time > 100) {
-      ESP_LOGE(TAG, "Reading from UART timed out at byte %u!", this->available());
+      ESP_LOGE(TAG, "Reading from UART timed out at byte %zu!", this->available());
       return false;
     }
     yield();