[uart] Fix available() return type to size_t across components (#13898)

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

esphome/components/modbus/modbus.cpp

@@ -20,10 +20,10 @@ void Modbus::loop() {
   const uint32_t now = App.get_loop_component_start_time();
 
   // Read all available bytes in batches to reduce UART call overhead.
-  int avail = this->available();
+  size_t avail = this->available();
   uint8_t buf[64];
   while (avail > 0) {
-    size_t to_read = std::min(static_cast<size_t>(avail), sizeof(buf));
+    size_t to_read = std::min(avail, sizeof(buf));
     if (!this->read_array(buf, to_read)) {
       break;
     }