[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_host.cpp

@@ -265,7 +265,7 @@ bool HostUartComponent::read_array(uint8_t *data, size_t len) {
   return true;
 }
 
-int HostUartComponent::available() {
+size_t HostUartComponent::available() {
   if (this->file_descriptor_ == -1) {
     return 0;
   }
@@ -275,9 +275,10 @@ int HostUartComponent::available() {
     this->update_error_(strerror(errno));
     return 0;
   }
+  size_t result = available;
   if (this->has_peek_)
-    available++;
-  return available;
+    result++;
+  return result;
 };
 
 void HostUartComponent::flush() {